metrics: Sink contructor from metric type

metrics/registry.go

@@ -77,20 +77,7 @@ func (r *Registry) newMetric(name string, mt MetricType, vt ...ValueType) *Metri
 		valueType = vt[0]
 	}
 
-	var sink Sink
-	switch mt {
-	case Counter:
-		sink = &CounterSink{}
-	case Gauge:
-		sink = &GaugeSink{}
-	case Trend:
-		sink = &TrendSink{}
-	case Rate:
-		sink = &RateSink{}
-	default:
-		return nil
-	}
-
+	sink := NewSink(mt)
 	return &Metric{
 		registry: r,
 		Name:     name,

metrics/sink.go

@@ -2,6 +2,7 @@ package metrics
 
 import (
 	"errors"
+	"fmt"
 	"math"
 	"sort"
 	"time"
@@ -21,6 +22,28 @@ type Sink interface {
 	IsEmpty() bool                             // Check if the Sink is empty.
 }
 
+// NewSink creates the related Sink for
+// the provided MetricType.
+func NewSink(mt MetricType) Sink {
+	var sink Sink
+	switch mt {
+	case Counter:
+		sink = &CounterSink{}
+	case Gauge:
+		sink = &GaugeSink{}
+	case Trend:
+		sink = &TrendSink{}
+	case Rate:
+		sink = &RateSink{}
+	default:
+		// Should not be possible to create
+		// an invalid metric type except for specific
+		// and controlled tests
+		panic(fmt.Sprintf("MetricType %q is not supported", mt))
+	}
+	return sink
+}
+
 type CounterSink struct {
 	Value float64
 	First time.Time

metrics/sink_test.go

@@ -9,6 +9,28 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestNewSink(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		sink interface{}
+		mt   MetricType
+	}{
+		{mt: Counter, sink: &CounterSink{}},
+		{mt: Gauge, sink: &GaugeSink{}},
+		{mt: Rate, sink: &RateSink{}},
+		{mt: Trend, sink: &TrendSink{}},
+	}
+	for _, tc := range tests {
+		assert.Equal(t, tc.sink, NewSink(tc.mt))
+	}
+}
+
+func TestNewSinkInvalidMetricType(t *testing.T) {
+	t.Parallel()
+	assert.Panics(t, func() { NewSink(MetricType(6)) })
+}
+
 func TestCounterSink(t *testing.T) {
 	samples10 := []float64{1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 100.0}
 	now := time.Now()