chore(llmobs): refactor out ragas base evaluator (#11846)

Creates a `RagasBaseEvaluator` class that ragas evaluators can share.
This is a split-out version of [this
PR](#11716) that has
incorporated changes based on the first round of comments from @Yun-Kim
and @Kyle-Verhoog

This class contains shared logic for:
- checking ragas dependencies are present, and throwing otherwise
- `submit_and_evaluate` function, which generally takes a span event,
calls evaluate to generate a score, and then submits an evaluation
- extracting out question, contexts, and answer from an LLM span

### Misc changes
- rename tests to specify they are faithfulness-specific tests
- throw when we parse an unsupported evaluator instead of logging a
warning
### 

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

---------

Co-authored-by: lievan <[email protected]>

ddtrace/llmobs/_evaluators/ragas/base.py

@@ -0,0 +1,213 @@
+import traceback
+from typing import List
+from typing import Optional
+from typing import Tuple
+from typing import Union
+
+from ddtrace.internal.logger import get_logger
+from ddtrace.internal.telemetry import telemetry_writer
+from ddtrace.internal.telemetry.constants import TELEMETRY_APM_PRODUCT
+from ddtrace.internal.telemetry.constants import TELEMETRY_LOG_LEVEL
+from ddtrace.internal.utils.version import parse_version
+from ddtrace.llmobs._constants import INTERNAL_CONTEXT_VARIABLE_KEYS
+from ddtrace.llmobs._constants import INTERNAL_QUERY_VARIABLE_KEYS
+from ddtrace.llmobs._constants import RAGAS_ML_APP_PREFIX
+
+
+logger = get_logger(__name__)
+
+
+class RagasDependencies:
+    """
+    A helper class to store instances of ragas classes and functions
+    that may or may not exist in a user's environment.
+    """
+
+    def __init__(self):
+        import ragas
+
+        self.ragas_version = parse_version(ragas.__version__)
+        if self.ragas_version >= (0, 2, 0) or self.ragas_version < (0, 1, 10):
+            raise NotImplementedError(
+                "Ragas version: {} is not supported".format(self.ragas_version),
+            )
+
+        from ragas.llms import llm_factory
+
+        self.llm_factory = llm_factory
+
+        from ragas.llms.output_parser import RagasoutputParser
+
+        self.RagasoutputParser = RagasoutputParser
+
+        from ragas.metrics import context_precision
+
+        self.context_precision = context_precision
+
+        from ragas.metrics.base import ensembler
+
+        self.ensembler = ensembler
+
+        from ragas.metrics import faithfulness
+
+        self.faithfulness = faithfulness
+
+        from ragas.metrics.base import get_segmenter
+
+        self.get_segmenter = get_segmenter
+
+        from ddtrace.llmobs._evaluators.ragas.models import StatementFaithfulnessAnswers
+
+        self.StatementFaithfulnessAnswers = StatementFaithfulnessAnswers
+
+        from ddtrace.llmobs._evaluators.ragas.models import StatementsAnswers
+
+        self.StatementsAnswers = StatementsAnswers
+
+
+def _get_ml_app_for_ragas_trace(span_event: dict) -> str:
+    """
+    The `ml_app` spans generated from traces of ragas will be named as `dd-ragas-<ml_app>`
+    or `dd-ragas` if `ml_app` is not present in the span event.
+    """
+    tags: List[str] = span_event.get("tags", [])
+    ml_app = None
+    for tag in tags:
+        if isinstance(tag, str) and tag.startswith("ml_app:"):
+            ml_app = tag.split(":")[1]
+            break
+    if not ml_app:
+        return RAGAS_ML_APP_PREFIX
+    return "{}-{}".format(RAGAS_ML_APP_PREFIX, ml_app)
+
+
+class BaseRagasEvaluator:
+    """A class used by EvaluatorRunner to conduct ragas evaluations
+    on LLM Observability span events. The job of an Evaluator is to take a span and
+    submit evaluation metrics based on the span's attributes.
+
+    Extenders of this class should only need to implement the `evaluate` method.
+    """
+
+    LABEL = "ragas"
+    METRIC_TYPE = "score"
+
+    def __init__(self, llmobs_service):
+        """
+        Initialize an evaluator that uses the ragas library to generate a score on finished LLM spans.
+
+        :param llmobs_service: An instance of the LLM Observability service used for tracing the evaluation and
+                                      submitting evaluation metrics.
+
+        Raises: NotImplementedError if the ragas library is not found or if ragas version is not supported.
+        """
+        self.llmobs_service = llmobs_service
+        self.ragas_version = "unknown"
+        telemetry_state = "ok"
+        try:
+            self.ragas_dependencies = RagasDependencies()
+            self.ragas_version = self.ragas_dependencies.ragas_version
+        except ImportError as e:
+            telemetry_state = "fail_import_error"
+            raise NotImplementedError("Failed to load dependencies for `{}` evaluator".format(self.LABEL)) from e
+        except AttributeError as e:
+            telemetry_state = "fail_attribute_error"
+            raise NotImplementedError("Failed to load dependencies for `{}` evaluator".format(self.LABEL)) from e
+        except NotImplementedError as e:
+            telemetry_state = "fail_not_supported"
+            raise NotImplementedError("Failed to load dependencies for `{}` evaluator".format(self.LABEL)) from e
+        except Exception as e:
+            telemetry_state = "fail_unknown"
+            raise NotImplementedError("Failed to load dependencies for `{}` evaluator".format(self.LABEL)) from e
+        finally:
+            telemetry_writer.add_count_metric(
+                namespace=TELEMETRY_APM_PRODUCT.LLMOBS,
+                name="evaluators.init",
+                value=1,
+                tags=(
+                    ("evaluator_label", self.LABEL),
+                    ("state", telemetry_state),
+                    ("evaluator_version", self.ragas_version),
+                ),
+            )
+            if telemetry_state != "ok":
+                telemetry_writer.add_log(
+                    level=TELEMETRY_LOG_LEVEL.ERROR,
+                    message="Failed to import Ragas dependencies",
+                    stack_trace=traceback.format_exc(),
+                    tags={"evaluator_version": self.ragas_version},
+                )
+
+    def run_and_submit_evaluation(self, span_event: dict):
+        if not span_event:
+            return
+        score_result_or_failure, metric_metadata = self.evaluate(span_event)
+        telemetry_writer.add_count_metric(
+            TELEMETRY_APM_PRODUCT.LLMOBS,
+            "evaluators.run",
+            1,
+            tags=(
+                ("evaluator_label", self.LABEL),
+                ("state", score_result_or_failure if isinstance(score_result_or_failure, str) else "success"),
+                ("evaluator_version", self.ragas_version),
+            ),
+        )
+        if isinstance(score_result_or_failure, float):
+            self.llmobs_service.submit_evaluation(
+                span_context={"trace_id": span_event.get("trace_id"), "span_id": span_event.get("span_id")},
+                label=self.LABEL,
+                metric_type=self.METRIC_TYPE,
+                value=score_result_or_failure,
+                metadata=metric_metadata,
+            )
+
+    def evaluate(self, span_event: dict) -> Tuple[Union[float, str], Optional[dict]]:
+        raise NotImplementedError("evaluate method must be implemented by individual evaluators")
+
+    def _extract_evaluation_inputs_from_span(self, span_event: dict) -> Optional[dict]:
+        """
+        Extracts the question, answer, and context used as inputs for a ragas evaluation on a span event.
+        """
+        with self.llmobs_service.workflow("dd-ragas.extract_evaluation_inputs_from_span") as extract_inputs_workflow:
+            self.llmobs_service.annotate(span=extract_inputs_workflow, input_data=span_event)
+            question, answer, contexts = None, None, None
+
+            meta_io = span_event.get("meta")
+            if meta_io is None:
+                return None
+
+            meta_input = meta_io.get("input")
+            meta_output = meta_io.get("output")
+
+            if not (meta_input and meta_output):
+                return None
+
+            prompt = meta_input.get("prompt")
+            if prompt is None:
+                logger.debug("Failed to extract `prompt` from span for ragas evaluation")
+                return None
+            prompt_variables = prompt.get("variables")
+
+            input_messages = meta_input.get("messages")
+
+            messages = meta_output.get("messages")
+            if messages is not None and len(messages) > 0:
+                answer = messages[-1].get("content")
+
+            if prompt_variables:
+                context_keys = prompt.get(INTERNAL_CONTEXT_VARIABLE_KEYS, ["context"])
+                question_keys = prompt.get(INTERNAL_QUERY_VARIABLE_KEYS, ["question"])
+                contexts = [prompt_variables.get(key) for key in context_keys if prompt_variables.get(key)]
+                question = " ".join([prompt_variables.get(key) for key in question_keys if prompt_variables.get(key)])
+
+            if not question and input_messages is not None and len(input_messages) > 0:
+                question = input_messages[-1].get("content")
+
+            self.llmobs_service.annotate(
+                span=extract_inputs_workflow, output_data={"question": question, "contexts": contexts, "answer": answer}
+            )
+            if any(field is None for field in (question, contexts, answer)):
+                logger.debug("Failed to extract inputs required for ragas evaluation")
+                return None
+
+            return {"question": question, "contexts": contexts, "answer": answer}