Add client ping stuff

src/client/Client.js

@@ -148,6 +148,13 @@ class Client extends EventEmitter {
      */
     this.readyAt = null;
 
+    /**
+     * The previous heartbeat pings of the websocket (most recent first, limited to three elements)
+     * @type {number[]}
+     */
+    this.pings = [];
+
+    this._pingTimestamp = 0;
     this._timeouts = new Set();
     this._intervals = new Set();
 
@@ -174,6 +181,15 @@ class Client extends EventEmitter {
     return this.readyAt ? Date.now() - this.readyAt : null;
   }
 
+  /**
+   * The average heartbeat ping of the websocket
+   * @type {number}
+   * @readonly
+   */
+  get ping() {
+    return this.pings.reduce((prev, p) => prev + p, 0) / this.pings.length;
+  }
+
   /**
    * Returns a collection, mapping guild ID to voice connections.
    * @type {Collection<string, VoiceConnection>}
@@ -392,6 +408,11 @@ class Client extends EventEmitter {
     this._intervals.delete(interval);
   }
 
+  _pong(startTime) {
+    this.pings.unshift(Date.now() - startTime);
+    if (this.pings.length > 3) this.pings.length = 3;
+  }
+
   _setPresence(id, presence) {
     if (this.presences.get(id)) {
       this.presences.get(id).update(presence);