feat: Adds a config.gatherStats parameter.

This PR exposes the functionality that's already built-in the
TraceablePeerConnection that has it gather stats from the wrapped
PeerConnection.

modules/RTC/TraceablePeerConnection.js

     this.statsinterval = null;
 
     /**
-     * @type {number}
+     * @type {number} The max number of stats to keep in this.stats. Limit to
+     * 300 values, i.e. 5 minutes; set to 0 to disable
      */
-    this.maxstats = 0;
+    this.maxstats = options.maxstats;
     const Interop = require('sdp-interop').Interop;
 
     this.interop = new Interop();

modules/xmpp/JingleSessionPC.js

  */
 const IQ_TIMEOUT = 10000;
 
+/*
+ * The default number of samples (per stat) to keep when webrtc stats gathering
+ * is enabled in TraceablePeerConnection.
+ */
+const DEFAULT_MAX_STATS = 300;
+
 /**
  *
  */
 
         const pcOptions = { disableRtx: this.room.options.disableRtx };
 
+        if (this.room.options.gatherStats) {
+            pcOptions.maxstats = DEFAULT_MAX_STATS;
+        }
+
         if (this.isP2P) {
             // simulcast needs to be disabled for P2P (121) calls
             pcOptions.disableSimulcast = true;