lib/httpserver: cache /metrics output for a second

This should reduce CPU load when `/metrics` output is scraped with a frequency exceeding a request per second

lib/httpserver/metrics.go

@@ -6,9 +6,12 @@ import (
 	"io"
 	"regexp"
 	"strings"
+	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/buildinfo"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/bytesutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/flagutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/memory"
 	"github.com/VictoriaMetrics/metrics"
@@ -18,6 +21,27 @@ var versionRe = regexp.MustCompile(`v\d+\.\d+\.\d+`)
 
 // WritePrometheusMetrics writes all the registered metrics to w in Prometheus exposition format.
 func WritePrometheusMetrics(w io.Writer) {
+	currentTime := time.Now()
+	metricsCacheLock.Lock()
+	if currentTime.Sub(metricsCacheLastUpdateTime) > time.Second {
+		var bb bytesutil.ByteBuffer
+		writePrometheusMetrics(&bb)
+		metricsCache.Store(&bb)
+		metricsCacheLastUpdateTime = currentTime
+	}
+	metricsCacheLock.Unlock()
+
+	bb := metricsCache.Load().(*bytesutil.ByteBuffer)
+	w.Write(bb.B)
+}
+
+var (
+	metricsCacheLock           sync.Mutex
+	metricsCacheLastUpdateTime time.Time
+	metricsCache               atomic.Value
+)
+
+func writePrometheusMetrics(w io.Writer) {
 	metrics.WritePrometheus(w, true)
 	metrics.WriteFDMetrics(w)