监控扩展

用户如果需要更详细的打点，例如包大小，或者想要更换其他数据源，例如 influxDB，用户可以根据自己的需求实现 ​Trace ​接口，并通过 ​WithTracer ​Option来注入。

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

从 ctx 中可以获得 RPCInfo，进一步的从 RPCInfo 中获取请求耗时、包大小和请求返回的错误信息等，举例：

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
}