app/vmauth: explicitly unregister metrics set for auth config

it's needed to remove Summary metric type from the global state of metrics package.
metrics package tracks each bucket of summary and periodically swaps old buckets with new.

Simple set unregister is not enough to release memory used by Set

#6247

app/vmauth/auth_config.go

@@ -693,6 +693,14 @@ func loadAuthConfig() (bool, error) {
 	authConfig.Store(ac)
 	authConfigData.Store(&data)
 	authUsers.Store(&m)
+	if prevAc != nil {
+		// explicilty unregister metrics, since all summary type metrics
+		// are registered at global state of metrics package
+		// and must be removed from it to release memory.
+		// Metrics must be unregistered only after atomic.Value.Store calls above
+		// Otherwise it may lead to metric gaps, since UnregisterAllMetrics is slow operation
+		prevAc.ms.UnregisterAllMetrics()
+	}
 
 	return true, nil
 }

docs/CHANGELOG.md

@@ -42,6 +42,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): fix bug that prevents the first query trace from expanding on click event. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6186). The issue was introduced in [v1.100.0](https://docs.victoriametrics.com/changelog/#v11000) release.
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent/): prevent potential panic during [stream aggregation](https://docs.victoriametrics.com/stream-aggregation.html) if more than one `--remoteWrite.streamAggr.dedupInterval` is configured. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6205).
 * BUGFIX: [stream aggregation](https://docs.victoriametrics.com/stream-aggregation/): set correct suffix `<output>_prometheus` for aggregation outputs [increase_prometheus](https://docs.victoriametrics.com/stream-aggregation/#increase_prometheus) and [total_prometheus](https://docs.victoriametrics.com/stream-aggregation/#total_prometheus). Before, outputs `total` and `total_prometheus` or `increase` and `increase_prometheus` had the same suffix.
+* BUGFIX: [vmauth](https://docs.victoriametrics.com/vmauth/): properly release memory used for metrics during config reload. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6247).
 
 ## [v1.101.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.101.0)