Metrics interface and Prometheus implementation (#1929)

* Metrics interface

* Prometheus implementation

* NoOp implementation

Co-authored-by: chris <chris@Profanity.local>

metrics/options_test.go

@@ -0,0 +1,18 @@
+package metrics
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestOptions(t *testing.T) {
+
+	// Make some new options:
+	options := NewOptions(Path("/prometheus"), DefaultTags(map[string]string{"service": "prometheus-test"}))
+
+	// Check that the defaults and overrides were accepted:
+	assert.Equal(t, ":9000", options.Address)
+	assert.Equal(t, "/prometheus", options.Path)
+	assert.Equal(t, "prometheus-test", options.DefaultTags["service"])
+}