Add Blink Prometheus listener (#11509)

bskylink/src/prometheus.ts

@@ -0,0 +1,54 @@
+import {collectDefaultMetrics, Gauge, Registry} from 'prom-client'
+
+import {type AppContext} from './context.js'
+
+let runtimeRegistry: Registry | undefined
+
+const getRuntimeRegistry = (): Registry => {
+  if (!runtimeRegistry) {
+    runtimeRegistry = new Registry()
+
+    // Beyla already exports HTTP RED metrics and traces for Blink. These
+    // process metrics cover the runtime-only failure modes it cannot see,
+    // particularly event-loop stalls, GC pauses, and V8 heap pressure.
+    collectDefaultMetrics({register: runtimeRegistry})
+  }
+
+  return runtimeRegistry
+}
+
+export const createPrometheusRegistry = (ctx: AppContext): Registry => {
+  const poolRegistry = new Registry()
+
+  new Gauge<'state'>({
+    name: 'bskylink_db_pool_connections',
+    help: 'PostgreSQL client connections by usage state.',
+    labelNames: ['state'],
+    registers: [poolRegistry],
+    collect() {
+      const {idleCount, totalCount} = ctx.db.cfg.pool
+      this.set({state: 'idle'}, idleCount)
+      this.set({state: 'in_use'}, totalCount - idleCount)
+    },
+  })
+
+  new Gauge({
+    name: 'bskylink_db_pool_max_connections',
+    help: 'Configured maximum PostgreSQL client connections.',
+    registers: [poolRegistry],
+    collect() {
+      this.set(ctx.cfg.db.pool.size)
+    },
+  })
+
+  new Gauge({
+    name: 'bskylink_db_pool_waiting_requests',
+    help: 'Requests waiting for a PostgreSQL client connection.',
+    registers: [poolRegistry],
+    collect() {
+      this.set(ctx.db.cfg.pool.waitingCount)
+    },
+  })
+
+  return Registry.merge([getRuntimeRegistry(), poolRegistry])
+}