Extract metrics to own package and refactor implementations

metrics/metrics.go

@@ -0,0 +1,79 @@
+package metrics
+
+import (
+	"github.com/go-kit/kit/metrics"
+	"github.com/go-kit/kit/metrics/multi"
+)
+
+// Registry has to implemented by any system that wants to monitor and expose metrics.
+type Registry interface {
+	// IsEnabled shows whether metrics instrumentation is enabled.
+	IsEnabled() bool
+	ReqsCounter() metrics.Counter
+	ReqDurationHistogram() metrics.Histogram
+	RetriesCounter() metrics.Counter
+}
+
+// NewMultiRegistry creates a new standardRegistry that wraps multiple Registries.
+func NewMultiRegistry(registries []Registry) Registry {
+	reqsCounters := []metrics.Counter{}
+	reqDurationHistograms := []metrics.Histogram{}
+	retriesCounters := []metrics.Counter{}
+
+	for _, r := range registries {
+		reqsCounters = append(reqsCounters, r.ReqsCounter())
+		reqDurationHistograms = append(reqDurationHistograms, r.ReqDurationHistogram())
+		retriesCounters = append(retriesCounters, r.RetriesCounter())
+	}
+
+	return &standardRegistry{
+		enabled:              true,
+		reqsCounter:          multi.NewCounter(reqsCounters...),
+		reqDurationHistogram: multi.NewHistogram(reqDurationHistograms...),
+		retriesCounter:       multi.NewCounter(retriesCounters...),
+	}
+}
+
+type standardRegistry struct {
+	enabled              bool
+	reqsCounter          metrics.Counter
+	reqDurationHistogram metrics.Histogram
+	retriesCounter       metrics.Counter
+}
+
+func (r *standardRegistry) IsEnabled() bool {
+	return r.enabled
+}
+
+func (r *standardRegistry) ReqsCounter() metrics.Counter {
+	return r.reqsCounter
+}
+
+func (r *standardRegistry) ReqDurationHistogram() metrics.Histogram {
+	return r.reqDurationHistogram
+}
+
+func (r *standardRegistry) RetriesCounter() metrics.Counter {
+	return r.retriesCounter
+}
+
+// NewVoidRegistry is a noop implementation of metrics.Registry.
+// It is used to avoid nil checking in components that do metric collections.
+func NewVoidRegistry() Registry {
+	return &standardRegistry{
+		enabled:              false,
+		reqsCounter:          &voidCounter{},
+		reqDurationHistogram: &voidHistogram{},
+		retriesCounter:       &voidCounter{},
+	}
+}
+
+type voidCounter struct{}
+
+func (v *voidCounter) With(labelValues ...string) metrics.Counter { return v }
+func (v *voidCounter) Add(delta float64)                          {}
+
+type voidHistogram struct{}
+
+func (h *voidHistogram) With(labelValues ...string) metrics.Histogram { return h }
+func (h *voidHistogram) Observe(value float64)                        {}