Make the debug metrics endpoints leader aware#192
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Pull request overview
This PR makes the debug metrics endpoints leader-aware by filtering out aggregated.* and probes.* metrics on follower nodes, preventing Datadog from observing stale/frozen expvar values after a leader demotion.
Changes:
- Replaces the old
/debug/varsand/debug/metricshandler with anAPIImpl.Metricsmethod that filtersaggregated.*andprobes.*when not leader. - Forces registry metrics to be (re)published into Go’s global
expvarregistry before rendering the filtered JSON response. - Adds a unit test to verify expvar values persist after unregister and are filtered on followers.
Show a summary per file
| File | Description |
|---|---|
| pkg/http/api.go | Adds a leader-aware Metrics handler and routes /debug/vars and /debug/metrics through it with filtering. |
| pkg/http/api_test.go | Adds coverage for leader vs follower behavior and expvar persistence after registry unregister. |
Review details
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- Files reviewed: 2/2 changed files
- Comments generated: 3
- Review effort level: Low
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| func (api *APIImpl) Metrics(w http.ResponseWriter, r *http.Request, _ httprouter.Params) { | ||
| exp.ExpHandler(metrics.DefaultRegistry).ServeHTTP(discardWriter{Writer: io.Discard}, r) | ||
| isLeader := api.consensusService != nil && api.consensusService.IsLeader() | ||
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| w.Header().Set("Content-Type", "application/json") |
ericyan
approved these changes
Jul 20, 2026
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This pull requests omits
aggregated.*andprobes.*metrics on followers.After a leader is demoted, active metric collection stops, but previously exported values remain in Go’s global expvar registry. Consequently, Datadog continues observing frozen aggregate gauges and zero-rate probe counters from the former leader.
Removing metrics from go-metrics does not remove values already published to expvar, so filtering is applied when rendering the metrics response.