Add counter for unaccessed failed promise warnings

[yeomin4242]

What changed

Add temporal_workflow_unaccessed_failed_promise, incremented once for each failed Promise reported by the existing unaccessed-promise warning during runner cleanup. Document the metric and add a changelog entry.

The counter follows the existing warning path, including cache eviction and Worker shutdown. Replay-time increments are suppressed by the existing metrics scope; another runner may report the same failure again.

Closes #2538.

[CLAassistant]

All committers have signed the CLA.

[Quinn-With-Two-Ns]

Thank you for the contribution, Just to be transparent we have not decided if we even want #2538 , how we want to support it and how it would effect other SDKs. These are things we need to discuss first before we would review this particular PR so we may not be able to review this PR immediately.

Is this a feature you would personally want or did you just want to contribute to the Java SDK?

[yeomin4242]

@Quinn-With-Two-Ns

Thanks for clarifying. I mainly wanted to contribute to the Java SDK and don’t have a specific production use case for this. I’ll keep the PR while the team decides how to approach this across SDKs.