PMM-9919 Add support for meta metrics

Signed-off-by: Shashank Sinha <[email protected]>

prometheus/collector_metric.go

@@ -0,0 +1,63 @@
+// Copyright 2022 Percona
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package prometheus
+
+var (
+	descScrapeTime *Desc
+	metricCache    chan Metric
+)
+
+type MetaMetrics struct {
+	scrapeTime  *Desc
+	metricCache chan Metric
+}
+
+func NewMetaMetricsCollector() Collector {
+	descScrapeTime = NewDesc(
+		"collector_scrape_time_ms",
+		"Time taken for scrape by collector",
+		[]string{"exporter", "collector"},
+		nil)
+
+	return &MetaMetrics{
+		scrapeTime:  descScrapeTime,
+		metricCache: makeMetricsCache(),
+	}
+}
+
+func makeMetricsCache() chan Metric {
+	// TODO : Use a more appropriate collector count
+	metricCache = make(chan Metric, 100)
+	return metricCache
+}
+
+func GetScrapeDescripter() *Desc {
+	return descScrapeTime
+}
+
+func PushMetaMetrics(m Metric) {
+	metricCache <- m
+}
+
+func (c *MetaMetrics) Describe(ch chan<- *Desc) {
+	ch <- c.scrapeTime
+}
+
+func (c *MetaMetrics) Collect(ch chan<- Metric) {
+	close(c.metricCache)
+	for m := range c.metricCache {
+		ch <- m
+	}
+	c.metricCache = makeMetricsCache()
+}

prometheus/registry.go

@@ -61,6 +61,7 @@ var (
 func init() {
 	MustRegister(NewProcessCollector(ProcessCollectorOpts{}))
 	MustRegister(NewGoCollector())
+	MustRegister(NewMetaMetricsCollector())
 }
 
 // NewRegistry creates a new vanilla Registry without any Collectors
@@ -446,11 +447,16 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 
 	wg.Add(goroutineBudget)
 
+	var metaMetricCollector Collector
 	collectWorker := func() {
 		for {
 			select {
 			case collector := <-checkedCollectors:
-				collector.Collect(checkedMetricChan)
+				if _, ok := collector.(*MetaMetrics); ok {
+					metaMetricCollector = collector
+				} else {
+					collector.Collect(checkedMetricChan)
+				}
 			case collector := <-uncheckedCollectors:
 				collector.Collect(uncheckedMetricChan)
 			default:
@@ -468,6 +474,9 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 	// are collected.
 	go func() {
 		wg.Wait()
+		if metaMetricCollector != nil {
+			metaMetricCollector.Collect(checkedMetricChan)
+		}
 		close(checkedMetricChan)
 		close(uncheckedMetricChan)
 	}()