Add feeds failing metric (#681)

* Track failing feeds in a metric

* Update docs

* changelog

changelog.d/681.misc

@@ -0,0 +1 @@
+Add `feed_failing` metric to track the number of feeds failing to be read or parsed.

docs/metrics.md

@@ -43,6 +43,7 @@ Below is the generated list of Prometheus metrics for Hookshot.
 |--------|------|--------|
 | feed_count | The number of RSS feeds that hookshot is subscribed to |  |
 | feed_fetch_ms | The time taken for hookshot to fetch all feeds |  |
+| feed_failing | The number of RSS feeds that hookshot is failing to read | reason |
 ## process
 | Metric | Help | Labels |
 |--------|------|--------|

src/Metrics.ts

@@ -26,6 +26,7 @@ export class Metrics {
 
     public readonly feedsCount = new Gauge({ name: "feed_count", help: "The number of RSS feeds that hookshot is subscribed to", labelNames: [], registers: [this.registry]});
     public readonly feedFetchMs = new Gauge({ name: "feed_fetch_ms", help: "The time taken for hookshot to fetch all feeds", labelNames: [], registers: [this.registry]});
+    public readonly feedsFailing = new Gauge({ name: "feed_failing", help: "The number of RSS feeds that hookshot is failing to read", labelNames: ["reason"], registers: [this.registry]});
 
 
     constructor(private registry: Registry = register) {

src/feeds/FeedReader.ts

@@ -104,6 +104,11 @@ export class FeedReader {
     private seenEntries: Map<string, string[]> = new Map();
     // A set of last modified times for each url.
     private cacheTimes: Map<string, { etag?: string, lastModified?: string}> = new Map();
+    
+    // Reason failures to url map.
+    private feedsFailingHttp = new Set();
+    private feedsFailingParsing = new Set();
+
     static readonly seenEntriesEventType = "uk.half-shot.matrix-hookshot.feed.reader.seenEntries";
 
     constructor(
@@ -199,6 +204,8 @@ export class FeedReader {
                     // We don't want to wait forever for the feed.
                     timeout: this.config.pollTimeoutSeconds * 1000,
                 });
+                // Clear any HTTP failures
+                this.feedsFailingHttp.delete(url);
                 
                 // Store any entity tags/cache times.
                 if (res.headers.ETag) {
@@ -208,6 +215,8 @@ export class FeedReader {
                 }
 
                 const feed = await this.parser.parseString(res.data);
+                this.feedsFailingParsing.delete(url);
+
                 let initialSync = false;
                 let seenGuids = this.seenEntries.get(url);
                 if (!seenGuids) {
@@ -268,6 +277,9 @@ export class FeedReader {
                     if (err.response?.status === StatusCodes.NOT_MODIFIED) {
                         continue;
                     }
+                    this.feedsFailingHttp.add(url);
+                } else {
+                    this.feedsFailingParsing.add(url);
                 }
                 const error = err instanceof Error ? err : new Error(`Unknown error ${err}`);
                 const feedError = new FeedError(url.toString(), error, fetchKey);
@@ -275,6 +287,10 @@ export class FeedReader {
                 this.queue.push<FeedError>({ eventName: 'feed.error', sender: 'FeedReader', data: feedError});
             }
         }
+
+        Metrics.feedsFailing.set({ reason: "http" }, this.feedsFailingHttp.size );
+        Metrics.feedsFailing.set({ reason: "parsing" }, this.feedsFailingParsing.size);
+
         if (seenEntriesChanged) await this.saveSeenEntries();
 
         const elapsed = Date.now() - fetchingStarted;