Add session id to traces and a new export endpoint to export traces for a session

[dineshyv]

What does this PR do?

• There is some issue with the decorator pattern of span creation when using async generators, where there are multiple yields with in the same context. I think its much more explicit by using the explicit context manager pattern using with. I moved the span creations in agent instance to be using with
• Inject session id at the turn level, which should quickly give us all traces across turns for a given session
• Adds a new endpoint to export traces based on session id

Addresses #509

Test Plan

llama stack run /Users/dineshyv/.llama/distributions/llamastack-together/together-run.yaml
PYTHONPATH=. python -m examples.agents.rag_with_memory_bank localhost 5000
curl "http://localhost:5000/alpha/telemetry/get-traces-for-session" \
-X POST \
-H "Content-Type: application/json" \
-d '{
    "session_id": "62c974d9-da34-4e3c-9831-d81e233882c4",
    "lookback": "2h",
    "limit": 100
}'  | jq

Output of export: https://pastebin.com/Y2mxMQXR

[yanxi0830]

Couple of questions:

1. Should we make lookback window to be int in second or smt? What does the limit mean?

2. Is the session_id specific to the Agent? Wondering how do we deal with llama-stack apps built with inference/chat_completion API call only?

[dineshyv]

1. The argument for lookback is actually a string for jaeger as well. I think its easier to specify the window using 1h or 5s instead of int.
2. If its not part of a session, we should be able to get the trace using just the trace id. We already have a endpoint for that, but not implemented for open telemetry. I can add that.

[dineshyv]

syned offline. We will be adding traces at the individial API level rather than focusing just on the agent.