✨ Allow overriding REST client latency histogram buckets

[wieghx]

What

Make classic histogram buckets for the opt-in REST client duration metrics overridable at registration time.

RegisterRESTClientMetrics is unchanged and still uses the Kubernetes-default buckets (starting at 5ms). Callers that scrape only classic buckets and need sub-5ms resolution can do:

metrics.RegisterRESTClientMetricsWithOptions(metrics.RESTClientMetricsOptions{
    DurationBuckets: []float64{0.001, 0.0025, 0.005, 0.025, 0.1, 0.25, 0.5, 1, 2, 4, 8, 15, 30, 60},
}, metrics.MetricRequestLatency, metrics.MetricRateLimiterLatency)

This applies to:

• rest_client_request_duration_seconds
• rest_client_dns_resolution_duration_seconds
• rest_client_rate_limiter_duration_seconds

Native histogram settings (NativeHistogramBucketFactor etc.) are not changed. Size histograms are not changed. The first RegisterRESTClientMetrics* call wins so already-registered collectors are not replaced.

Why

Follow-up to #3510 / #3559. Fast controller hot-path PATCH/PUT latencies often sit in the 1–15ms range; with the default 5ms / 25ms buckets, p50/p90 on classic histograms is mostly quantization artifact. Changing the defaults would diverge from Kubernetes core, so this is opt-in only.

How to verify

go test ./pkg/metrics/ -count=1

This PR was written in part with the assistance of generative AI.

Fixes #3559

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: wieghx / name: wieghx (65d8c00)

[kubernetes-prow]

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[kubernetes-prow]

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[kubernetes-prow]

Hi @wieghx. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[wieghx]

CLA is signed (cncf-cla: yes). Happy to address review comments.

Could an org member please add /ok-to-test when you get a chance?

[sbueringer]

/ok-to-test