Add HeartbeatLatency method (bwmarrin#593)

* Latency method

* fixed typo

* fixed linter error

* Renamed Latency to HeartbeatLatency

* HeartbeatLatency now returns time.Time

* return time.Duration instead, since .Sub() returns that

* Add full-stops to end of comments

structs.go

@@ -85,6 +85,9 @@ type Session struct {
 	// Stores the last HeartbeatAck that was recieved (in UTC)
 	LastHeartbeatAck time.Time
 
+	// Stores the last Heartbeat sent (in UTC)
+	LastHeartbeatSent time.Time
+
 	// used to deal with rate limits
 	Ratelimiter *RateLimiter
 

wsapi.go

@@ -267,6 +267,13 @@ type helloOp struct {
 // FailedHeartbeatAcks is the Number of heartbeat intervals to wait until forcing a connection restart.
 const FailedHeartbeatAcks time.Duration = 5 * time.Millisecond
 
+// HeartbeatLatency returns the latency between heartbeat acknowledgement and heartbeat send.
+func (s *Session) HeartbeatLatency() time.Duration {
+
+	return s.LastHeartbeatAck.Sub(s.LastHeartbeatSent)
+
+}
+
 // heartbeat sends regular heartbeats to Discord so it knows the client
 // is still connected.  If you do not send these heartbeats Discord will
 // disconnect the websocket connection after a few seconds.
@@ -289,6 +296,7 @@ func (s *Session) heartbeat(wsConn *websocket.Conn, listening <-chan interface{}
 		sequence := atomic.LoadInt64(s.sequence)
 		s.log(LogDebug, "sending gateway websocket heartbeat seq %d", sequence)
 		s.wsMutex.Lock()
+		s.LastHeartbeatSent = time.Now().UTC()
 		err = wsConn.WriteJSON(heartbeatOp{1, sequence})
 		s.wsMutex.Unlock()
 		if err != nil || time.Now().UTC().Sub(last) > (heartbeatIntervalMsec*FailedHeartbeatAcks) {