Metrics interface and Prometheus implementation (#1929)

* Metrics interface

* Prometheus implementation

* NoOp implementation

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

metrics/noop/reporter_test.go

@@ -0,0 +1,20 @@
+package noop
+
+import (
+	"testing"
+
+	"github.com/micro/go-micro/v3/metrics"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNoopReporter(t *testing.T) {
+
+	// Make a Reporter:
+	reporter := New(metrics.Path("/noop"))
+	assert.NotNil(t, reporter)
+	assert.Equal(t, "/noop", reporter.options.Path)
+
+	// Check that our implementation is valid:
+	assert.Implements(t, new(metrics.Reporter), reporter)
+}