Make the metadata field any

[lowjiansheng]

A similar PR may already be submitted!
Please search among the Pull request before creating one.

If your changes introduce breaking changes, please prefix the title of your pull request with "[BREAKING_CHANGES]". This allows for clear identification of such changes in the 'What's Changed' section on the release page, making it developer-friendly.

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request.

Describe the change
For some providers like LiteLLM, metadata fields can be arrays. By strictly typing it to string, these metadata fields will not be accepted and parsed appropriately.
If we make it any, we can pass in strings too for backward compatibility.

Tests
Briefly describe how you have tested these changes. If possible — please add integration tests.

Additional context
Add any other context or screenshots or logs about your pull request here. If the pull request relates to an open issue, please link to it.

Issue: #XXXX

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.93%. Comparing base (4d2e7ab) to head (b897c34).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1000   +/-   ##
=======================================
  Coverage   85.93%   85.93%           
=======================================
  Files          43       43           
  Lines        2268     2268           
=======================================
  Hits         1949     1949           
  Misses        300      300           
  Partials       19       19           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.