add feature to extra the last qpsum

Summary:
extracts the qpSum which is a quality metrics for encoded images. See
  https://w3c.github.io/webrtc-stats/#dom-rtcinboundrtpstreamstats-qpsum

Dividing this by the number of seconds a stream is active gives us an idea about the average quality

Test Plan:
- run extract.js with a dump
- observe qpsum feature

Reviewers: gustavo

Reviewed By: gustavo

Differential Revision: https://lifeonair.phacility.com/D25260

features-stream.js

@@ -33,6 +33,13 @@ module.exports = {
         }
         return duration;
     },
+    qpSum: ({kind, stats}) => {
+        if (kind !== 'video' || !stats.length) {
+            return;
+        }
+        const last = stats[stats.length - 1];
+        return last.qpSum;
+    },
 };
 
 /* these features operate on stats of each track, in send and recv direction */

features-v2.sql

@@ -259,6 +259,8 @@ CREATE TABLE features (
     videogoogbandwidthlimitedresolutionmin real,
     videogoogbandwidthlimitedresolutionmode real,
 
+    qpsum real,
+
     audiocodec character varying(64),
     videocodec character varying(64),