code: func (si *serverInterceptor) observeLatency(clientReqTime string) error {
  	// Convert the metadata request time into an int64
  	reqTimeUnixNanos, err := strconv.ParseInt(clientReqTime, 10, 64)
  	if err != nil {
  		return berrors.InternalServerError("grpc metadata had illegal %s value: %q - %s",
  			clientRequestTimeKey, clientReqTime, err)
  	}
  	// Calculate the elapsed time since the client sent the RPC
  	reqTime := time.Unix(0, reqTimeUnixNanos)
  	elapsed := si.clk.Since(reqTime)
  	// Publish an RPC latency observation to the histogram
  	si.metrics.rpcLag.Observe(elapsed.Seconds())
  	return nil
  }
docs: // observeLatency is called with the `clientRequestTimeKey` value from
  // a request's gRPC metadata. This string value is converted to a timestamp and
  // used to calcuate the latency between send and receive time. The latency is
  // published to the server interceptor's rpcLag prometheus histogram. An error
  // is returned if the `clientReqTime` string is not a valid timestamp.