Expose voice connection ping in player stats. (#738)

* expose gateway ping in player stats

* document ping in IMPLEMENTATION.md

Author: devoxin

IMPLEMENTATION.md

@@ -251,14 +251,16 @@ This event includes:
 * Unix timestamp in milliseconds.
 * Track position in milliseconds. Omitted if not playing anything.
 * `connected` is true when connected to the voice gateway.
+* `ping` represents the number of milliseconds between heartbeat and ack. Could be `-1` if not connected.
 ```json
 {
     "op": "playerUpdate",
     "guildId": "...",
     "state": {
         "time": 1500467109,
         "position": 60000,
-        "connected": true
+        "connected": true,
+        "ping": 0
     }
 }
 ```

LavalinkServer/src/main/java/lavalink/server/io/SocketServer.kt

@@ -61,8 +61,9 @@ class SocketServer(
             val json = JSONObject()
 
             val state = player.state
-            val connected = socketContext.getMediaConnection(player).gatewayConnection?.isOpen == true
-            state.put("connected", connected)
+            val connection = socketContext.getMediaConnection(player).gatewayConnection
+            state.put("connected", connection?.isOpen == true)
+            state.put("ping", connection?.ping ?: -1)
 
             json.put("op", "playerUpdate")
             json.put("guildId", player.guildId.toString())