feat: Make size_limit (max run count per batch) globally configurable…

… via env var (#1358)

### Purpose
Compression improves run throughput and memory usage, but performance is
still slower than expected for power users due to excessive threads and
small payloads. SDK users can improve the throughput for larger payloads
and higher trace counts by making the run count batch ingest config
globally configurable.

### Changes
* Created an environment variable `LANGSMITH_BATCH_INGEST_SIZE_LIMIT` to
configure run count.

* This takes precedence over the `/info` api response and any value
passed directly to the client batch ingest config.

### Tests / Benchmarks
Setting a higher run count increased compressed runs throughput by
**~80-100%** depending on run count and payload size.

python/langsmith/_internal/_background_thread.py

@@ -18,6 +18,7 @@
 )
 
 from langsmith import schemas as ls_schemas
+from langsmith import utils as ls_utils
 from langsmith._internal._constants import (
     _AUTO_SCALE_DOWN_NEMPTY_TRIGGER,
     _AUTO_SCALE_UP_NTHREADS_LIMIT,
@@ -157,6 +158,21 @@ def _tracing_thread_handle_batch(
             tracing_queue.task_done()
 
 
+def get_size_limit_from_env() -> Optional[int]:
+    size_limit_str = ls_utils.get_env_var(
+        "BATCH_INGEST_SIZE_LIMIT",
+    )
+    if size_limit_str is not None:
+        try:
+            return int(size_limit_str)
+        except ValueError:
+            logger.warning(
+                f"Invalid value for BATCH_INGEST_SIZE_LIMIT: {size_limit_str}, "
+                "continuing with default"
+            )
+    return None
+
+
 def _ensure_ingest_config(
     info: ls_schemas.LangSmithInfo,
 ) -> ls_schemas.BatchIngestConfig:
@@ -173,6 +189,9 @@ def _ensure_ingest_config(
     try:
         if not info.batch_ingest_config:
             return default_config
+        env_size_limit = get_size_limit_from_env()
+        if env_size_limit is not None:
+            info.batch_ingest_config["size_limit"] = env_size_limit
         return info.batch_ingest_config
     except BaseException:
         return default_config