VertexAI emit user, system, and assistant events

[aabmass]

Description

Part of #3041, follow up to #3192

Adds Gen AI events for model input to the VertexAI instrumentation. These are system event, user event, and assistant event.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Added test cases for the various log events with OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT varied true and false.

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[xrmx]

Not sure log emitting should be governed by span recording

[aabmass]

This is copied from openai-v2:

opentelemetry-python-contrib/instrumentation-genai/opentelemetry-instrumentation-openai-v2/src/opentelemetry/instrumentation/openai_v2/patch.py

Lines 59 to 63 in 2756c1e

	if span.is_recording():
	for message in kwargs.get("messages", []):
	event_logger.emit(
	message_to_event(message, capture_content)
	)

I can see the merit both ways, created #3207 to discuss in GenAI SIG

cc @lmolkova

[drewby]

We are currently making an implicit assumption that events are recorded as part of a span, but the specification does not explicitly require this—though all the examples at the end show events within spans. It might be worth updating the specification to clarify this expectation either way.