fix(propagation): make baggage header extract case insensitive (#11859)

Following the WSGI spec, headers shouldn't be case sensitive, therefore
baggage, like the other headers we extract, should not be case
sensitive.
https://peps.python.org/pep-3333/#environ-variables: "(A reminder for
server/gateway authors: HTTP header names are case-insensitive, so be
sure to take that into consideration when examining application-supplied
headers!)"

## 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
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[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

ddtrace/propagation/http.py

@@ -101,6 +101,7 @@ def _possible_header(header):
 _POSSIBLE_HTTP_HEADER_B3_FLAGS = _possible_header(_HTTP_HEADER_B3_FLAGS)
 _POSSIBLE_HTTP_HEADER_TRACEPARENT = _possible_header(_HTTP_HEADER_TRACEPARENT)
 _POSSIBLE_HTTP_HEADER_TRACESTATE = _possible_header(_HTTP_HEADER_TRACESTATE)
+_POSSIBLE_HTTP_BAGGAGE_HEADER = _possible_header(_HTTP_HEADER_BAGGAGE)
 
 
 # https://www.w3.org/TR/trace-context/#traceparent-header-field-values
@@ -937,7 +938,7 @@ def _inject(span_context: Context, headers: Dict[str, str]) -> None:
 
     @staticmethod
     def _extract(headers: Dict[str, str]) -> Context:
-        header_value = headers.get(_HTTP_HEADER_BAGGAGE)
+        header_value = _extract_header_value(_POSSIBLE_HTTP_BAGGAGE_HEADER, headers)
 
         if not header_value:
             return Context(baggage={})

releasenotes/notes/case_insensitive_baggage_header_extraction-63167c492474da6f.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    tracer: This fix resolves an issue where baggage header extraction was case sensitive and didn't accept the header prepended with HTTP.
+    Now the baggage header will be extracted regardless of casing and the HTTP format.
+    

tests/tracer/test_propagation.py

@@ -1888,6 +1888,14 @@ def test_extract_tracecontext(headers, expected_context):
         B3_SINGLE_HEADERS_VALID,
         CONTEXT_EMPTY,
     ),
+    (
+        "baggage_case_insensitive",
+        None,
+        {"BAgGage": "key1=val1,key2=val2"},
+        {
+            "baggage": {"key1": "val1", "key2": "val2"},
+        },
+    ),
     # All valid headers
     (
         "valid_all_headers_default_style",
@@ -2278,14 +2286,14 @@ def test_propagation_extract_w_config(name, styles, headers, expected_context, r
     overrides = {}
     if styles is not None:
         overrides["_propagation_style_extract"] = styles
-        with override_global_config(overrides):
-            context = HTTPPropagator.extract(headers)
-            if not expected_context.get("tracestate"):
-                assert context == Context(**expected_context)
-            else:
-                copied_expectation = expected_context.copy()
-                tracestate = copied_expectation.pop("tracestate")
-                assert context == Context(**copied_expectation, meta={"tracestate": tracestate})
+    with override_global_config(overrides):
+        context = HTTPPropagator.extract(headers)
+        if not expected_context.get("tracestate"):
+            assert context == Context(**expected_context)
+        else:
+            copied_expectation = expected_context.copy()
+            tracestate = copied_expectation.pop("tracestate")
+            assert context == Context(**copied_expectation, meta={"tracestate": tracestate})
 
 
 EXTRACT_OVERRIDE_FIXTURES = [