Gauge to track the current number of metrics known [was: metricsCount metric remain the same after metric been deleted by RemoveStaleMetrics]

[LeoQuote]

I was using

statsd_exporter/main.go

Lines 167 to 173 in 58769c7

	metricsCount = promauto.NewGaugeVec(
	prometheus.GaugeOpts{
	Name: "statsd_exporter_metrics_total",
	Help: "The total number of metrics.",
	},
	[]string{"type"},
	)

to track the metric number of statsd exporter, but found that this metric never drops, even if I set the ttl parameter in config file.

Then I noticed, across all the reference of the metrics, these's no place to decrease the gauge.

even in the

statsd_exporter/pkg/registry/registry.go

Lines 381 to 396 in 58769c7

	func (r *Registry) RemoveStaleMetrics() {
	now := clock.Now()
	// delete timeseries with expired ttl
	for _, metric := range r.Metrics {
	for hash, rm := range metric.Metrics {
	if rm.TTL == 0 {
	continue
	}
	if rm.LastRegisteredAt.Add(rm.TTL).Before(now) {
	metric.Vectors[rm.VecKey].Holder.Delete(rm.Labels)
	metric.Vectors[rm.VecKey].RefCount--
	delete(metric.Metrics, hash)
	}
	}
	}
	}

So I think if we add a decrease in this function, the metric number would show the real number of metric number

[matthiasr]

This is a counter, it represents the total number of metrics seen by the exporter, and by definition counters cannot decrease. I think what you are looking for is a metric for the current number of metrics the exporter knows about, which would have to be a separate gauge. Let's call it statsd_exporter_metrics_current or something? I'm happy to take a contribution to add that.

[LeoQuote]

The metric name looks good to me, thanks for your contribution in advance!

[SuperQ]

@matthiasr Isn't that already captured by scrape_samples_scraped and scrape_samples_post_metric_relabeling?

[LeoQuote]

@SuperQ the original metric statsd_exporter_metrics_total provides separated counters for each metric type, including counter, gauge, and observer, this could be useful

[yuminn-k]

Opened PR #715 to add statsd_exporter_metrics_current as a per-type gauge for the current number of active metric vectors.

I kept statsd_exporter_metrics_total as the cumulative counter and update the new gauge when vectors become active or expire via TTL cleanup.