telemetry: Refactor GRPC Server Handler (#756)

* Refactor GRPC server stats handler location

internal/telemetry/grpc.go

@@ -0,0 +1,39 @@
+package telemetry
+
+import (
+	"context"
+
+	"go.opencensus.io/plugin/ocgrpc"
+	grpcstats "google.golang.org/grpc/stats"
+
+	"github.com/pomerium/pomerium/internal/telemetry/metrics"
+)
+
+type tagRPCHandler interface {
+	TagRPC(context.Context, *grpcstats.RPCTagInfo) context.Context
+}
+
+// GRPCServerStatsHandler provides a grpc stats.Handler for metrics and tracing for a pomerium service
+type GRPCServerStatsHandler struct {
+	service        string
+	metricsHandler tagRPCHandler
+	grpcstats.Handler
+}
+
+// TagRPC implements grpc.stats.Handler and adds metrics and tracing metadata to the context of a given RPC
+func (h *GRPCServerStatsHandler) TagRPC(ctx context.Context, tagInfo *grpcstats.RPCTagInfo) context.Context {
+
+	metricCtx := h.metricsHandler.TagRPC(ctx, tagInfo)
+	handledCtx := h.Handler.TagRPC(metricCtx, tagInfo)
+
+	return handledCtx
+}
+
+// NewGRPCServerStatsHandler creates a new GRPCServerStatsHandler for a pomerium service
+func NewGRPCServerStatsHandler(service string) grpcstats.Handler {
+	return &GRPCServerStatsHandler{
+		service:        service,
+		Handler:        &ocgrpc.ServerHandler{},
+		metricsHandler: metrics.NewGRPCServerMetricsHandler(service),
+	}
+}