Add relay connections metrics

src/controllers/metrics.ts

@@ -1,17 +1,32 @@
 import { register } from 'prom-client';
 
 import { AppController } from '@/app.ts';
-import { dbAvailableConnectionsGauge, dbPoolSizeGauge } from '@/metrics.ts';
+import {
+  dbAvailableConnectionsGauge,
+  dbPoolSizeGauge,
+  relayPoolRelaysSizeGauge,
+  relayPoolSubscriptionsSizeGauge,
+} from '@/metrics.ts';
 import { Storages } from '@/storages.ts';
 
 /** Prometheus/OpenMetrics controller. */
 export const metricsController: AppController = async (c) => {
   const db = await Storages.database();
+  const pool = await Storages.client();
 
   // Update some metrics at request time.
   dbPoolSizeGauge.set(db.poolSize);
   dbAvailableConnectionsGauge.set(db.availableConnections);
 
+  relayPoolRelaysSizeGauge.reset();
+  relayPoolSubscriptionsSizeGauge.reset();
+
+  for (const relay of pool.relays.values()) {
+    relayPoolRelaysSizeGauge.inc({ ready_state: relay.socket.readyState });
+    relayPoolSubscriptionsSizeGauge.inc(relay.subscriptions.length);
+  }
+
+  // Serve the metrics.
   const metrics = await register.metrics();
 
   const headers: HeadersInit = {

src/metrics.ts

@@ -119,3 +119,14 @@ export const internalSubscriptionsSizeGauge = new Gauge({
   name: 'ditto_internal_subscriptions_size',
   help: "Number of active subscriptions to Ditto's internal relay",
 });
+
+export const relayPoolRelaysSizeGauge = new Gauge({
+  name: 'ditto_relay_pool_relays_size',
+  help: 'Number of relays in the relay pool',
+  labelNames: ['ready_state'],
+});
+
+export const relayPoolSubscriptionsSizeGauge = new Gauge({
+  name: 'ditto_relay_pool_subscriptions_size',
+  help: 'Number of active subscriptions to the relay pool',
+});