Add debug trace endpoint in api server

Details:
- Add a in-memory SpanExporter to capture all trace information.
- Add /debug/trace/session/{session_id} endpoint to retrieve traces from the in-memory exporter.
- Add Session ID in Telemetry spans.

PiperOrigin-RevId: 757984565

src/google/adk/cli/fast_api.py

@@ -48,6 +48,7 @@ from opentelemetry.exporter.cloud_trace import CloudTraceSpanExporter
 from opentelemetry.sdk.trace import export
 from opentelemetry.sdk.trace import ReadableSpan
 from opentelemetry.sdk.trace import TracerProvider
+from opentelemetry.sdk.trace.export.in_memory_span_exporter import InMemorySpanExporter
 from pydantic import alias_generators
 from pydantic import BaseModel
 from pydantic import ConfigDict
@@ -112,6 +113,42 @@ class ApiServerSpanExporter(export.SpanExporter):
     return True
 
 
+class InMemoryExporter(export.SpanExporter):
+
+  def __init__(self, trace_dict):
+    super().__init__()
+    self._spans = []
+    self.trace_dict = trace_dict
+
+  def export(
+      self, spans: typing.Sequence[ReadableSpan]
+  ) -> export.SpanExportResult:
+    for span in spans:
+      trace_id = span.context.trace_id
+      if span.name == "call_llm":
+        attributes = dict(span.attributes)
+        session_id = attributes.get("gcp.vertex.agent.session_id", None)
+        if session_id:
+          if session_id not in self.trace_dict:
+            self.trace_dict[session_id] = [trace_id]
+          else:
+            self.trace_dict[session_id] += [trace_id]
+    self._spans.extend(spans)
+    return export.SpanExportResult.SUCCESS
+
+  def get_finished_spans(self, session_id: str):
+    trace_ids = self.trace_dict.get(session_id, None)
+    if trace_ids is None or not trace_ids:
+      return []
+    return [x for x in self._spans if x.context.trace_id in trace_ids]
+
+  def force_flush(self, timeout_millis: int = 30000) -> bool:
+    return True
+
+  def clear(self):
+    self._spans.clear()
+
+
 class AgentRunRequest(BaseModel):
   app_name: str
   user_id: str
@@ -152,12 +189,15 @@ def get_fast_api_app(
 ) -> FastAPI:
   # InMemory tracing dict.
   trace_dict: dict[str, Any] = {}
+  session_trace_dict: dict[str, Any] = {}
 
   # Set up tracing in the FastAPI server.
   provider = TracerProvider()
   provider.add_span_processor(
       export.SimpleSpanProcessor(ApiServerSpanExporter(trace_dict))
   )
+  memory_exporter = InMemoryExporter(session_trace_dict)
+  provider.add_span_processor(export.SimpleSpanProcessor(memory_exporter))
   if trace_to_cloud:
     envs.load_dotenv_for_agent("", agent_dir)
     if project_id := os.environ.get("GOOGLE_CLOUD_PROJECT", None):
@@ -254,6 +294,24 @@ def get_fast_api_app(
       raise HTTPException(status_code=404, detail="Trace not found")
     return event_dict
 
+  @app.get("/debug/trace/session/{session_id}")
+  def get_session_trace(session_id: str) -> Any:
+    spans = memory_exporter.get_finished_spans(session_id)
+    if not spans:
+      return []
+    return [
+        {
+            "name": s.name,
+            "span_id": s.context.span_id,
+            "trace_id": s.context.trace_id,
+            "start_time": s.start_time,
+            "end_time": s.end_time,
+            "attributes": dict(s.attributes),
+            "parent_span_id": s.parent.span_id if s.parent else None,
+        }
+        for s in spans
+    ]
+
   @app.get(
       "/apps/{app_name}/users/{user_id}/sessions/{session_id}",
       response_model_exclude_none=True,
@@ -306,7 +364,6 @@ def get_fast_api_app(
       raise HTTPException(
           status_code=400, detail=f"Session already exists: {session_id}"
       )
-
     logger.info("New session created: %s", session_id)
     return session_service.create_session(
         app_name=app_name, user_id=user_id, state=state, session_id=session_id
@@ -323,7 +380,6 @@ def get_fast_api_app(
   ) -> Session:
     # Connect to managed session if agent_engine_id is set.
     app_name = agent_engine_id if agent_engine_id else app_name
-
     logger.info("New session created")
     return session_service.create_session(
         app_name=app_name, user_id=user_id, state=state

src/google/adk/telemetry.py

@@ -111,6 +111,9 @@ def trace_call_llm(
   span.set_attribute(
       'gcp.vertex.agent.invocation_id', invocation_context.invocation_id
   )
+  span.set_attribute(
+      'gcp.vertex.agent.session_id', invocation_context.session.id
+  )
   span.set_attribute('gcp.vertex.agent.event_id', event_id)
   # Consider removing once GenAI SDK provides a way to record this info.
   span.set_attribute(