inbound and outbound requests are in different spans（带内和带外的请求在不同的span中）
spans that include cs can log an sa annotation of where they are going（包含cs的span可以用sa标记他们请求的地址）
This helps when the destination protocol isn’t Zipkin instrumented, such as MySQL.
cs - Client Send. The client has made the request. This sets the beginning of the span.
sr - Server Receive: The server has received the request and will start processing it. The difference between this and cs will be combination of network latency and clock jitter.
ss - Server Send: The server has completed processing and has sent the request back to the client. The difference between this and sr will be the amount of time it took the server to process the request.
cr - Client Receive: The client has received the response from the server. This sets the end of the span. The RPC is considered complete when this annotation is recorded.