监控扩展

// Tracer is executed at the start and finish of an RPC.
type Tracer interface {
    Start(ctx context.Context) context.Context
    Finish(ctx context.Context)
}

type clientTracer struct {
    // contain entities which recording metric
}

// Start record the beginning of an RPC invocation.
func (c *clientTracer) Start(ctx context.Context) context.Context {
    // do nothing
	return ctx
}

// Finish record after receiving the response of server.
func (c *clientTracer) Finish(ctx context.Context) {
	ri := rpcinfo.GetRPCInfo(ctx)
	rpcStart := ri.Stats().GetEvent(stats.RPCStart)
	rpcFinish := ri.Stats().GetEvent(stats.RPCFinish)
	cost := rpcFinish.Time().Sub(rpcStart.Time())
	// TODO: record the cost of request
}