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/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(