code: func (c *clientHandler) TagRPC(ctx context.Context, rti *stats.RPCTagInfo) context.Context {
  	var span zipkin.Span
  
  	ep := remoteEndpointFromContext(ctx, c.remoteServiceName)
  
  	name := spanName(rti)
  	span, ctx = c.tracer.StartSpanFromContext(ctx, name, zipkin.Kind(model.Client), zipkin.RemoteEndpoint(ep))
  
  	md, ok := metadata.FromOutgoingContext(ctx)
  	if ok {
  		md = md.Copy()
  	} else {
  		md = metadata.New(nil)
  	}
  	_ = b3.InjectGRPC(&md)(span.Context())
  	ctx = metadata.NewOutgoingContext(ctx, md)
  	return ctx
  }
docs: // TagRPC implements per-RPC context management.