webhook: add StatsReporterOptions in webhook.Options

There are two ways to customize StatsReporter:

1. Use a whole new StatsReporter implementation.
1. Or pass Option funcs to customize the default StatsReporter.

Option 1 is less practical at this time due to the metrics registration
conflict. `webhook.RegisterMetrics()` is called regardless which
StatsReporter implementation is used (which is a problem by itself). The
second option is more practical since it works well without dealing with
metrics conflicts.

The `webhook.Option` in particular allows people to discard certain
metrics tags.

webhook/webhook.go

@@ -70,6 +70,9 @@ type Options struct {
 	// only a single port for the service.
 	Port int
 
+	// StatsReporterOptions are the options used to initialize the default StatsReporter
+	StatsReporterOptions []Option
+
 	// StatsReporter reports metrics about the webhook.
 	// This will be automatically initialized by the constructor if left uninitialized.
 	StatsReporter StatsReporter
@@ -144,7 +147,7 @@ func New(
 	logger := logging.FromContext(ctx)
 
 	if opts.StatsReporter == nil {
-		reporter, err := NewStatsReporter()
+		reporter, err := NewStatsReporter(opts.StatsReporterOptions...)
 		if err != nil {
 			return nil, err
 		}