fix(opik): enhance requester metadata retrieval from API key auth

[Thomas-Mildner]

Title

Automatically read Opik metadata from the user's API key when not provided in the request.
If no metadata is stored for the API key, default to an empty value (maintaining previous behavior).

Relevant issues

Feature Proposal for #15895

Pre-Submission checklist

Please complete all items before asking a LiteLLM maintainer to review your PR

• I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
• I have added a screenshot of my new test passing locally
• My PR passes all unit tests on make test-unit
• My PR's scope is as isolated as possible, it only solves 1 specific problem

Type

🆕 New Feature

Changes

[vercel]

@Thomas-Mildner is attempting to deploy a commit to the CLERKIEAI Team on Vercel.

A member of the Team first needs to authorize it.

[ghost]

@Thomas-Mildner can you please document this behaviour?

[Thomas-Mildner]

@krrishdholakia Sure, i created a new PR with the corresponding documentation.
#16004