feat(monitoring): add Prometheus metrics endpoint and Grafana dashboards

Add observability stack with @willsoto/nestjs-prometheus for /metrics endpoint,
Prometheus scraping config, and 4 auto-provisioned Grafana dashboards
(API overview, database, search, business metrics).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/app.module.ts

@@ -6,6 +6,7 @@ import { NotificationsModule } from '@modules/notifications';
 import { PaymentsModule } from '@modules/payments';
 import { SubscriptionsModule } from '@modules/subscriptions';
 import { AdminModule } from '@modules/admin';
+import { MetricsModule } from '@modules/metrics';
 import { Module } from '@nestjs/common';
 import { APP_GUARD } from '@nestjs/core';
 import { CqrsModule } from '@nestjs/cqrs';
@@ -24,6 +25,7 @@ import { AppController } from './app.controller';
     PaymentsModule,
     SubscriptionsModule,
     AdminModule,
+    MetricsModule,
 
     // ── Rate Limiting ──
     // Default: 60 requests per 60 seconds per IP

apps/api/src/modules/metrics/index.ts

@@ -0,0 +1 @@
+export { MetricsModule } from './metrics.module';

apps/api/src/modules/metrics/metrics.module.ts

@@ -0,0 +1,69 @@
+import { Module } from '@nestjs/common';
+import {
+  PrometheusModule,
+  makeCounterProvider,
+  makeHistogramProvider,
+  makeGaugeProvider,
+} from '@willsoto/nestjs-prometheus';
+
+@Module({
+  imports: [
+    PrometheusModule.register({
+      path: '/metrics',
+      defaultMetrics: { enabled: true },
+    }),
+  ],
+  providers: [
+    // ── HTTP Metrics ──
+    makeHistogramProvider({
+      name: 'http_request_duration_seconds',
+      help: 'Duration of HTTP requests in seconds',
+      labelNames: ['method', 'route', 'status_code'],
+      buckets: [0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10],
+    }),
+    makeCounterProvider({
+      name: 'http_requests_total',
+      help: 'Total number of HTTP requests',
+      labelNames: ['method', 'route', 'status_code'],
+    }),
+
+    // ── Database Metrics ──
+    makeHistogramProvider({
+      name: 'db_query_duration_seconds',
+      help: 'Duration of database queries in seconds',
+      labelNames: ['operation', 'model'],
+      buckets: [0.001, 0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5],
+    }),
+    makeGaugeProvider({
+      name: 'db_pool_active_connections',
+      help: 'Number of active database connections',
+    }),
+
+    // ── Search Metrics ──
+    makeHistogramProvider({
+      name: 'search_query_duration_seconds',
+      help: 'Duration of search queries in seconds',
+      labelNames: ['collection', 'type'],
+      buckets: [0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1],
+    }),
+
+    // ── Business Metrics ──
+    makeCounterProvider({
+      name: 'listings_created_total',
+      help: 'Total number of listings created',
+      labelNames: ['category'],
+    }),
+    makeCounterProvider({
+      name: 'payments_processed_total',
+      help: 'Total number of payments processed',
+      labelNames: ['status', 'method'],
+    }),
+    makeGaugeProvider({
+      name: 'active_subscriptions',
+      help: 'Number of active subscriptions',
+      labelNames: ['plan'],
+    }),
+  ],
+  exports: [PrometheusModule],
+})
+export class MetricsModule {}