Change `gen_ai.response.finish_reasons` type from array to `string`, use it on metrics

[lmolkova]

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Fixes #1277

Changes

While OpenAI supports returning multiple completions (and therefore could have multiple finish reasons), I did not find models outside of OpenAI that return multiple choices and many SDKs (e.g. vercel) don't support returning multiple choices.

So effectively there is usually one finish reason. Having an array for it:

• complicates querying
• makes usage of finish reason on metrics more complicated

This PR:

• turns finish reasons attribute into a string (comma-separated if there is more than one)
• adds it to metrics.

Having comma-separated list has high-ish theoretical cardinality in edge cases, but results in best experience for common case. I'm suggesting to start with it and consider making it opt-in on metrics if it's proven to be problematic.

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [ ] Change log entry added, according to the guidelines in When to add a changelog entry.
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• [ ] schema-next.yaml updated with changes to existing conventions.