[FEATURE]: add OTEL integration

[dtfiedler]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

A solid PR has already been submitted with #10652

Enable OTEL within opencode server to trace sessions and events.

[github-actions]

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This issue might be a duplicate of or closely related to existing issues. Please check:

• Add OTEL export capabilities for observability and monitoring #12142: Add OTEL export capabilities for observability and monitoring

If you believe this issue is distinct from existing work, please clarify how this differs from #12142.

[dtfiedler]

I lied - there are already open issues. Will link those in the PR.