Add prometheus gauge metric for OBO cache (hit/miss/expired)

server/src/auth/cache-gauge.ts

@@ -0,0 +1,11 @@
+import { proxyRegister } from '@app/prometheus/types';
+import { Gauge } from 'prom-client';
+
+const labelNames = ['hit'] as const;
+
+export const cacheGauge = new Gauge({
+  name: 'obo_cache',
+  help: 'Number of requests to the OBO cache. "hit" is the type of hit: "miss", "hit", or "expired".',
+  labelNames,
+  registers: [proxyRegister],
+});

server/src/auth/cache.ts

@@ -1,3 +1,4 @@
+import { cacheGauge } from '@app/auth/cache-gauge';
 import { getLogger } from '@app/logger';
 
 const log = getLogger('obo-cache');
@@ -19,17 +20,22 @@ export class OboMemoryCache {
     const value = this.cache.get(key);
 
     if (value === undefined) {
+      cacheGauge.inc({ hit: 'miss' });
+
       return null;
     }
 
     const [token, expiresAt] = value;
 
     if (expiresAt <= now()) {
+      cacheGauge.inc({ hit: 'expired' });
       this.cache.delete(key);
 
       return null;
     }
 
+    cacheGauge.inc({ hit: 'hit' });
+
     return token;
   }