Add grpc prometheus metrics

This provideds generic grpc metrics via prometheus

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/containerd/builtins.go

@@ -6,6 +6,7 @@ import (
 	_ "github.com/docker/containerd/services/content"
 	_ "github.com/docker/containerd/services/execution"
 	_ "github.com/docker/containerd/services/healthcheck"
+	_ "github.com/docker/containerd/services/metrics"
 	_ "github.com/docker/containerd/snapshot/btrfs"
 	_ "github.com/docker/containerd/snapshot/overlay"
 )