prometheus: Add Counter interface

Add a Counter interface that is a subset of the Prometheus Counter type
as well as a MockCounter that can be used by tests.

2 files changed, 32 insertions, 0 deletions

diff --git a/internal/prometheus/metrics/metrics.go b/internal/prometheus/metrics/metrics.go
index 9ca576de6..62b72d374 100644
--- a/internal/prometheus/metrics/metrics.go
+++ b/internal/prometheus/metrics/metrics.go
@@ -4,6 +4,12 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
+// Counter is a subset of a prometheus Counter
+type Counter interface {
+	Inc()
+	Add(float64)
+}
+
 // Gauge is a subset of a prometheus Gauge
 type Gauge interface {
 	Inc()
diff --git a/internal/testhelper/promtest/counter.go b/internal/testhelper/promtest/counter.go
new file mode 100644
index 000000000..665023436
--- /dev/null
+++ b/internal/testhelper/promtest/counter.go
@@ -0,0 +1,26 @@
+package promtest
+
+import (
+	"sync"
+)
+
+type MockCounter struct {
+	m     sync.RWMutex
+	value float64
+}
+
+func (m *MockCounter) Value() float64 {
+	m.m.RLock()
+	defer m.m.RUnlock()
+	return m.value
+}
+
+func (m *MockCounter) Inc() {
+	m.Add(1)
+}
+
+func (m *MockCounter) Add(v float64) {
+	m.m.Lock()
+	defer m.m.Unlock()
+	m.value += v
+}