Using attributes for chat history on gen_ai spans and events

[lmolkova]

Fixes #2010, #2179, #1913, #1621, #1912

Depends on #2588

Key points:

1. System message is recorded as gen_ai.system.instructions attribute. It's not necessarily a part of chat history (when dealing with server-side agents)
2. Prompt is recorded as gen_ai.input.messages attribute, format is defined in json schema (for now until we have tooling to define complex attributes). If uploading to an external storage, it's replaced with gen_ai.input.messages_ref
3. Completion is recorded as gen_ai.output.messages (or gen_ai.output.messages_ref)
4. These attributes can be recorded on spans or on the new event gen_ai.client.inference.operation.details which captures all the details of the inference operation so that it can be useful if span is sampled out and/or if queries between spans and logs backends is problematic.

Depends on #2046 (gen_ai.system rename and migration plan)

[aabmass]

Thanks for putting this together! What's need to take this out of draft?

[zhirafovod]

Unfortunately had to skip today's meeting because of a conflict. Wanted to bring up some cases for sending the prompts and messages as events:

1. Some customers want to send this customer-sensitive data to a different backend, for compliance reasons.
2. this data is useful independently from application tracing or metrics, as it provides an option to build a conversation-specific tools and UIs.
3. tracing usually implemented with different backend technologies

All of this is specifically important for non-purposely built solution, but for a bigger observability platforms.

@lmolkova My proposal would be to keep the events optional, and keep them in the existing implementations enabled with an environment variable.

[aabmass]

Here is a rough prototype of this PR for opentelemetry-instrumentation-google-genai open-telemetry/opentelemetry-python-contrib@main...aabmass:opentelemetry-python-contrib:google-genai-message_ref

[alexmojaki]

Thanks for all your hard work and diligence @lmolkova !

[jsuereth]

I wonder if we need first-class interaction with the JSON schema + weaver (e.g. live-check).

@lmolkova Have you already opened tickets in Weaver to make sure it also moves the same direction for supporting events here? (i.e. the 'nested' json schema approach), or are you considering something else going forward?