fix(blooms): improves mempool metrics (#13283)

Author: owen-d

pkg/util/mempool/metrics.go

@@ -8,13 +8,19 @@ import (
 )
 
 type metrics struct {
-	availableBuffersPerSlab *prometheus.CounterVec
+	availableBuffersPerSlab *prometheus.GaugeVec
 	errorsCounter           *prometheus.CounterVec
+	accesses                *prometheus.CounterVec
 }
 
+const (
+	opTypeGet = "get"
+	opTypePut = "put"
+)
+
 func newMetrics(r prometheus.Registerer, name string) *metrics {
 	return &metrics{
-		availableBuffersPerSlab: promauto.With(r).NewCounterVec(prometheus.CounterOpts{
+		availableBuffersPerSlab: promauto.With(r).NewGaugeVec(prometheus.GaugeOpts{
 			Namespace:   constants.Loki,
 			Subsystem:   "mempool",
 			Name:        "available_buffers_per_slab",
@@ -28,5 +34,12 @@ func newMetrics(r prometheus.Registerer, name string) *metrics {
 			Help:        "The total amount of errors returned from the pool.",
 			ConstLabels: prometheus.Labels{"pool": name},
 		}, []string{"slab", "reason"}),
+		accesses: promauto.With(r).NewCounterVec(prometheus.CounterOpts{
+			Namespace:   constants.Loki,
+			Subsystem:   "mempool",
+			Name:        "accesses_total",
+			Help:        "The total amount of accesses to the pool.",
+			ConstLabels: prometheus.Labels{"pool": name},
+		}, []string{"slab", "op"}),
 	}
 }

pkg/util/mempool/pool.go

@@ -27,7 +27,7 @@ type slab struct {
 
 func newSlab(bufferSize, bufferCount int, m *metrics) *slab {
 	name := humanize.Bytes(uint64(bufferSize))
-	m.availableBuffersPerSlab.WithLabelValues(name).Add(0) // initialize metric with value 0
+	m.availableBuffersPerSlab.WithLabelValues(name).Set(0) // initialize metric with value 0
 
 	return &slab{
 		size:    bufferSize,
@@ -44,10 +44,11 @@ func (s *slab) init() {
 		ptr := unsafe.Pointer(unsafe.SliceData(buf))
 		s.buffer <- ptr
 	}
-	s.metrics.availableBuffersPerSlab.WithLabelValues(s.name).Add(float64(s.count))
+	s.metrics.availableBuffersPerSlab.WithLabelValues(s.name).Set(float64(s.count))
 }
 
 func (s *slab) get(size int) ([]byte, error) {
+	s.metrics.accesses.WithLabelValues(s.name, opTypeGet).Inc()
 	s.mtx.Lock()
 	if s.buffer == nil {
 		s.init()
@@ -77,6 +78,7 @@ func (s *slab) get(size int) ([]byte, error) {
 }
 
 func (s *slab) put(buf []byte) {
+	s.metrics.accesses.WithLabelValues(s.name, opTypePut).Inc()
 	if s.buffer == nil {
 		panic("slab is not initialized")
 	}