code: func NewServerHandler(tracer *zipkin.Tracer, options ...ServerOption) stats.Handler {
  	c := &serverHandler{
  		tracer: tracer,
  	}
  	for _, option := range options {
  		option(c)
  	}
  	return c
  }
docs: // NewServerHandler returns a stats.Handler which can be used with grpc.WithStatsHandler to add
  // tracing to a gRPC server. The gRPC method name is used as the span name and by default the only
  // tags are the gRPC status code if the call fails. Use ServerTags to add additional tags that
  // should be applied to all spans.