Add metric for yt cached result age

package.json

@@ -2,7 +2,7 @@
   "author": "Calvin Montgomery",
   "name": "CyTube",
   "description": "Online media synchronizer and chat",
-  "version": "3.69.0",
+  "version": "3.70.0",
   "repository": {
     "url": "http://github.com/calzoneman/sync"
   },

src/database/metadata_cache.js

@@ -1,6 +1,8 @@
+import { Summary } from 'prom-client';
 import { createMySQLDuplicateKeyUpdate } from '../util/on-duplicate-key-update';
 
 const Media = require('cytube-mediaquery/lib/media');
+const LOGGER = require('@calzoneman/jsli')('metadata-cache');
 
 // TODO: these fullname-vs-shortcode hacks really need to be abolished
 function mediaquery2cytube(type) {
@@ -21,6 +23,11 @@ function cytube2mediaquery(type) {
     }
 }
 
+const cachedResultAge = new Summary({
+    name: 'cytube_yt_cache_result_age_seconds',
+    help: 'Age (in seconds) of cached record'
+});
+
 class MetadataCacheDB {
     constructor(db) {
         this.db = db;
@@ -55,6 +62,15 @@ class MetadataCacheDB {
                 return null;
             }
 
+            try {
+                let age = (Date.now() - row.updated_at.getTime())/1000;
+                if (age > 0) {
+                    cachedResultAge.observe(age);
+                }
+            } catch (error) {
+                LOGGER.error('Failed to record cachedResultAge metric: %s', error.stack);
+            }
+
             let metadata = JSON.parse(row.metadata);
             metadata.type = cytube2mediaquery(metadata.type);
             return new Media(metadata);