Coverage gaps in `opentelemetry-instrumentation-google-genai` instrumentations

[DylanRussell]

Using this to track some gaps as I go about monkey patching various APIs as part of the openinference donation stuff..

Gaps:
A generic way for the user to provide attributes that make it into the span.. We use a ContextVar to do this for the GenerateContent method, and could do the same for the other APIs..

Embeddings:

• We could allow capturing various EmbedContentConfig in attributes the same way we do GenerateContentConfig in the generate_content instrumentation.
• Openinference captures the actual raw embeddings in their instrumentation -- that seems too big to put into an attribute..
• google-genai SDK EmbedContentResponse Discards usageMetadata and Leaves sdk_http_response.body as None googleapis/python-genai#2658 -- EmbedContentResponse has a bug that doesn't let us get tokens from usage_metadata

Interactions:

• Need to capture interaction ID: Add new sem conv request attribute gen_ai.request.previous_response.id semantic-conventions-genai#372
• Expand input / output messages to capture all inputs / outputs -- the SDK accepts so many different types (related sem conv issue: Decide how to capture model / agent evaluation steps semantic-conventions-genai#380)