Include more request configuration options into the span attributes for the Google GenAI SDK instrumentation

[michaelsafyan]

Description

Improves the comprehensiveness and uniformity of reporting span-level attributes for Google GenAI SDK operations.

This essentially dumps all of the configuration options into span attributes, with a vendor-specific prefix. The subset of request options that are defined in OTel Semantic Conventions are then renamed to match conventions.

In service of the above, this change also:

1. Adds a flatten_dict util used to generate the span attributes.
2. Adds tests to more fully cover the set of attributes that are included.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Added more integration tests, covering the relevant changes.

Does This PR Require a Core Repo Change?

• No.

Checklist:

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

[michaelsafyan]

Friendly ping. Is it possible to get this reviewed?

[michaelsafyan]

There is still conversation in SemConv PR 2125 (open-telemetry/semantic-conventions#2125) related to defining the Semantic Conventions for this change. However, I think it would be good to get this in at this time and then circle back if additional changes are required to minimize risk of merge conflicts / bit rotting.