telemetry: Refactor GRPC Server Handler (#756)

* Refactor GRPC server stats handler location

internal/telemetry/grpc_test.go

@@ -0,0 +1,37 @@
+package telemetry
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"go.opencensus.io/plugin/ocgrpc"
+	grpcstats "google.golang.org/grpc/stats"
+)
+
+type mockTagHandler struct {
+	called bool
+}
+
+type mockCtxTag string
+
+func (m *mockTagHandler) TagRPC(ctx context.Context, tagInfo *grpcstats.RPCTagInfo) context.Context {
+	m.called = true
+	return context.WithValue(ctx, mockCtxTag("added"), "true")
+}
+
+func Test_GRPCServerStatsHandler(t *testing.T) {
+
+	metricsHandler := &mockTagHandler{}
+	h := &GRPCServerStatsHandler{
+		metricsHandler: metricsHandler,
+		Handler:        &ocgrpc.ServerHandler{},
+	}
+
+	ctx := context.WithValue(context.Background(), mockCtxTag("original"), "true")
+	ctx = h.TagRPC(ctx, &grpcstats.RPCTagInfo{})
+
+	assert.True(t, metricsHandler.called)
+	assert.Equal(t, ctx.Value(mockCtxTag("added")), "true")
+	assert.Equal(t, ctx.Value(mockCtxTag("original")), "true")
+}