docs: add circuit breaker metrics counters

[mfkd]

Summary

• document the three XR circuit breaker Prometheus counters alongside the existing Crossplane metrics table

Related issues

• Fixes crossplane/crossplane#6850
• PR crossplane/crossplane#6854

Testing

• I ran hugo server and ensured it formats correctly

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[jbw976]

Thanks for the docs update too @mfkd!

• looks like the DCO check is failing, can you make sure the commit(s) are all signed off?
  • https://github.com/crossplane/crossplane/tree/main/contributing#certificate-of-origin
• do these metrics names need to be updated after this refactor PR has now been merged?
  • crossplane/crossplane#6870

[mfkd]

Thanks @jbw976. I’ve pushed your requested changes.

Namely:

1. DSO sign off
2. Updated the circuit breaker metrics from crossplane_circuit_breaker_* to circuit_breaker_* to reflect the refactor changes in crossplane/crossplane#6870

While reviewing the metrics docs, I noticed gaps around engine, function-runner, and cached-runner metrics. The docs appear to list only one histogram metric and omit related counters/gauges, which may make it harder for users to discover all available metrics. Is this intentional for now?

Code references:
• Engine metrics — not currently listed in metrics.md
• Function runner metrics — only run_function_seconds histogram appears to be documented. Also the documentation has not been updated to reflect the naming change made here
• Cached runner metrics — not currently listed in metrics.md

From what I can tell, the documented metrics mainly cover those exposed by controller-runtime, client-go, and some implemented in crossplane-runtime but omits much of the metrics mentioned above in crossplane/crossplane.

Would it make sense to open follow-up PRs to document these missing metrics in metrics.md, or is there existing work covering this?