Enable grpc timing histograms

This enables the grpc timing histograms via a config option as they are
metrics of high cardinality.

This is useful for perf testing and debugging but should not be the
default on production systems unless needed.

```toml
[metrics]
	grpc_histogram = true

```

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

server/config.go

@@ -50,7 +50,8 @@ type Debug struct {
 
 // MetricsConfig provides metrics configuration
 type MetricsConfig struct {
-	Address string `toml:"address"`
+	Address       string `toml:"address"`
+	GRPCHistogram bool   `toml:"grpc_histogram"`
 }
 
 // CgroupConfig provides cgroup configuration

server/server.go

@@ -70,6 +70,7 @@ func New(ctx context.Context, config *Config) (*Server, error) {
 		s        = &Server{
 			rpc:    rpc,
 			events: exchange.NewExchange(),
+			config: config,
 		}
 		initialized = plugin.NewPluginSet()
 	)
@@ -127,10 +128,15 @@ func New(ctx context.Context, config *Config) (*Server, error) {
 type Server struct {
 	rpc    *grpc.Server
 	events *exchange.Exchange
+	config *Config
 }
 
 // ServeGRPC provides the containerd grpc APIs on the provided listener
 func (s *Server) ServeGRPC(l net.Listener) error {
+	if s.config.Metrics.GRPCHistogram {
+		// enable grpc time histograms to measure rpc latencies
+		grpc_prometheus.EnableHandlingTimeHistogram()
+	}
 	// before we start serving the grpc API regster the grpc_prometheus metrics
 	// handler.  This needs to be the last service registered so that it can collect
 	// metrics for every other service