Handle duplicate prometheus metrics registrations (#192)

Handle adding prometheus metrics in the same way that honeycomb metrics class works.

Surprised that no one else has run into this!

Another thing we could do here in addition to this: we could append the dataset that the sampler is for to the metrics so that they should all be unique

metrics/prometheus.go

@@ -42,7 +42,16 @@ func (p *PromMetrics) Start() error {
 // Register takes a name and a metric type. The type should be one of "counter",
 // "gauge", or "histogram"
 func (p *PromMetrics) Register(name string, metricType string) {
-	var newmet interface{}
+	p.lock.Lock()
+	defer p.lock.Unlock()
+
+	newmet, exists := p.metrics[name]
+
+	// don't attempt to add the metric again as this will cause a panic
+	if exists {
+		return
+	}
+
 	switch metricType {
 	case "counter":
 		newmet = promauto.NewCounter(prometheus.CounterOpts{
@@ -61,9 +70,7 @@ func (p *PromMetrics) Register(name string, metricType string) {
 		})
 	}
 
-	p.lock.Lock()
 	p.metrics[name] = newmet
-	p.lock.Unlock()
 }
 
 func (p *PromMetrics) IncrementCounter(name string) {

metrics/prometheus_test.go

@@ -0,0 +1,26 @@
+// +build all race
+
+package metrics
+
+import (
+	"testing"
+
+	"github.com/honeycombio/refinery/config"
+	"github.com/honeycombio/refinery/logger"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMultipleRegistrations(t *testing.T) {
+	p := &PromMetrics{
+		Logger: &logger.MockLogger{},
+		Config: &config.MockConfig{},
+	}
+
+	err := p.Start()
+
+	assert.NoError(t, err)
+
+	p.Register("test", "counter")
+
+	p.Register("test", "counter")
+}