keeper: expose Prometheus-style metrics endpoint

[karagozemin]

Summary

Add a /metrics endpoint to the keeper exposing round-processing counters, settle latency, and error counts in a scrapeable text format.

Motivation

Hosted keeper operations (SCF tranche 3) need observability; today only ad-hoc logs exist.

Acceptance criteria

• /metrics returns counters for rounds seen/revealed/settled/failed and latency histograms.
• Endpoint is opt-in via config.
• Test asserting metric names and increments.
• Documented in the keeper README.

[ifygreg01-best]

I have an experience with this issue
I would love to work on it, already reviewed the issue please assign me on this

[Topmatrixmor2014]

I would love to work on this issue

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #161)
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