feat(metrics): add MetricsService, HttpMetricsInterceptor, and metric constants

- Extract metric names into constants with goodgo_ prefix for business metrics
- Add MetricsService for type-safe metric recording
- Add HttpMetricsInterceptor for automatic request duration/count tracking
- Register interceptor globally via APP_INTERCEPTOR
- Include linter auto-fixes for test files

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

apps/api/src/app.module.ts

@@ -1,5 +1,5 @@
 import { type MiddlewareConsumer, Module, type NestModule } from '@nestjs/common';
-import { APP_FILTER, APP_GUARD } from '@nestjs/core';
+import { APP_FILTER, APP_GUARD, APP_INTERCEPTOR } from '@nestjs/core';
 import { CqrsModule } from '@nestjs/cqrs';
 import { ThrottlerModule } from '@nestjs/throttler';
 import { SentryGlobalFilter, SentryModule } from '@sentry/nestjs/setup';
@@ -8,7 +8,7 @@ import { AnalyticsModule } from '@modules/analytics';
 import { AuthModule } from '@modules/auth';
 import { ListingsModule } from '@modules/listings';
 import { McpIntegrationModule } from '@modules/mcp';
-import { MetricsModule } from '@modules/metrics';
+import { HttpMetricsInterceptor, MetricsModule } from '@modules/metrics';
 import { NotificationsModule } from '@modules/notifications';
 import { PaymentsModule } from '@modules/payments';
 import { SearchModule } from '@modules/search';
@@ -68,6 +68,10 @@ import { AppController } from './app.controller';
       provide: APP_GUARD,
       useClass: ThrottlerBehindProxyGuard,
     },
+    {
+      provide: APP_INTERCEPTOR,
+      useClass: HttpMetricsInterceptor,
+    },
   ],
 })
 export class AppModule implements NestModule {

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

@@ -1 +1,14 @@
 export { MetricsModule } from './metrics.module';
+export { MetricsService } from './infrastructure/metrics.service';
+export { HttpMetricsInterceptor } from './presentation/interceptors/http-metrics.interceptor';
+export {
+  GOODGO_LISTINGS_CREATED_TOTAL,
+  GOODGO_PAYMENTS_PROCESSED_TOTAL,
+  GOODGO_ACTIVE_SUBSCRIPTIONS,
+  GOODGO_SEARCH_QUERIES_TOTAL,
+  GOODGO_API_REQUEST_DURATION,
+  HTTP_REQUESTS_TOTAL,
+  DB_QUERY_DURATION,
+  DB_POOL_ACTIVE_CONNECTIONS,
+  SEARCH_QUERY_DURATION,
+} from './metrics.constants';

apps/api/src/modules/metrics/infrastructure/metrics.service.ts

@@ -0,0 +1,65 @@
+import { Injectable } from '@nestjs/common';
+import { InjectMetric } from '@willsoto/nestjs-prometheus';
+import { type Counter, type Gauge, type Histogram } from 'prom-client';
+import {
+  GOODGO_LISTINGS_CREATED_TOTAL,
+  GOODGO_PAYMENTS_PROCESSED_TOTAL,
+  GOODGO_ACTIVE_SUBSCRIPTIONS,
+  GOODGO_SEARCH_QUERIES_TOTAL,
+  GOODGO_API_REQUEST_DURATION,
+  HTTP_REQUESTS_TOTAL,
+} from '../metrics.constants';
+
+@Injectable()
+export class MetricsService {
+  constructor(
+    @InjectMetric(GOODGO_LISTINGS_CREATED_TOTAL)
+    private readonly listingsCreatedCounter: Counter,
+    @InjectMetric(GOODGO_PAYMENTS_PROCESSED_TOTAL)
+    private readonly paymentsProcessedCounter: Counter,
+    @InjectMetric(GOODGO_ACTIVE_SUBSCRIPTIONS)
+    private readonly activeSubscriptionsGauge: Gauge,
+    @InjectMetric(GOODGO_SEARCH_QUERIES_TOTAL)
+    private readonly searchQueriesCounter: Counter,
+    @InjectMetric(GOODGO_API_REQUEST_DURATION)
+    private readonly requestDurationHistogram: Histogram,
+    @InjectMetric(HTTP_REQUESTS_TOTAL)
+    private readonly httpRequestsCounter: Counter,
+  ) {}
+
+  /** Record a new listing creation. */
+  recordListingCreated(category: string): void {
+    this.listingsCreatedCounter.inc({ category });
+  }
+
+  /** Record a payment processing event. */
+  recordPaymentProcessed(status: string, method: string): void {
+    this.paymentsProcessedCounter.inc({ status, method });
+  }
+
+  /** Set the current number of active subscriptions for a plan tier. */
+  setActiveSubscriptions(plan: string, count: number): void {
+    this.activeSubscriptionsGauge.set({ plan }, count);
+  }
+
+  /** Record a search query. */
+  recordSearchQuery(collection: string, type: string): void {
+    this.searchQueriesCounter.inc({ collection, type });
+  }
+
+  /** Record HTTP request duration and count (used by interceptor). */
+  recordHttpRequest(
+    method: string,
+    route: string,
+    statusCode: number,
+    durationSeconds: number,
+  ): void {
+    const labels = {
+      method,
+      route,
+      status_code: String(statusCode),
+    };
+    this.requestDurationHistogram.observe(labels, durationSeconds);
+    this.httpRequestsCounter.inc(labels);
+  }
+}

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

@@ -0,0 +1,12 @@
+// ── Business Metrics (goodgo_ prefix) ──
+export const GOODGO_LISTINGS_CREATED_TOTAL = 'goodgo_listings_created_total';
+export const GOODGO_PAYMENTS_PROCESSED_TOTAL = 'goodgo_payments_processed_total';
+export const GOODGO_ACTIVE_SUBSCRIPTIONS = 'goodgo_active_subscriptions';
+export const GOODGO_SEARCH_QUERIES_TOTAL = 'goodgo_search_queries_total';
+export const GOODGO_API_REQUEST_DURATION = 'goodgo_api_request_duration_seconds';
+
+// ── Infrastructure Metrics ──
+export const HTTP_REQUESTS_TOTAL = 'http_requests_total';
+export const DB_QUERY_DURATION = 'db_query_duration_seconds';
+export const DB_POOL_ACTIVE_CONNECTIONS = 'db_pool_active_connections';
+export const SEARCH_QUERY_DURATION = 'search_query_duration_seconds';

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

@@ -5,6 +5,19 @@ import {
   makeHistogramProvider,
   makeGaugeProvider,
 } from '@willsoto/nestjs-prometheus';
+import { MetricsService } from './infrastructure/metrics.service';
+import {
+  GOODGO_API_REQUEST_DURATION,
+  GOODGO_LISTINGS_CREATED_TOTAL,
+  GOODGO_PAYMENTS_PROCESSED_TOTAL,
+  GOODGO_ACTIVE_SUBSCRIPTIONS,
+  GOODGO_SEARCH_QUERIES_TOTAL,
+  HTTP_REQUESTS_TOTAL,
+  DB_QUERY_DURATION,
+  DB_POOL_ACTIVE_CONNECTIONS,
+  SEARCH_QUERY_DURATION,
+} from './metrics.constants';
+import { HttpMetricsInterceptor } from './presentation/interceptors/http-metrics.interceptor';
 
 @Module({
   imports: [
@@ -16,56 +29,63 @@ import {
   providers: [
     // ── HTTP Metrics ──
     makeHistogramProvider({
-      name: 'http_request_duration_seconds',
+      name: GOODGO_API_REQUEST_DURATION,
       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',
+      name: HTTP_REQUESTS_TOTAL,
       help: 'Total number of HTTP requests',
       labelNames: ['method', 'route', 'status_code'],
     }),
 
     // ── Database Metrics ──
     makeHistogramProvider({
-      name: 'db_query_duration_seconds',
+      name: DB_QUERY_DURATION,
       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',
+      name: DB_POOL_ACTIVE_CONNECTIONS,
       help: 'Number of active database connections',
     }),
 
     // ── Search Metrics ──
     makeHistogramProvider({
-      name: 'search_query_duration_seconds',
+      name: SEARCH_QUERY_DURATION,
       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],
     }),
-
-    // ── Cache Metrics ── (registered in SharedModule alongside CacheService)
+    makeCounterProvider({
+      name: GOODGO_SEARCH_QUERIES_TOTAL,
+      help: 'Total number of search queries',
+      labelNames: ['collection', 'type'],
+    }),
 
     // ── Business Metrics ──
     makeCounterProvider({
-      name: 'listings_created_total',
+      name: GOODGO_LISTINGS_CREATED_TOTAL,
       help: 'Total number of listings created',
       labelNames: ['category'],
     }),
     makeCounterProvider({
-      name: 'payments_processed_total',
+      name: GOODGO_PAYMENTS_PROCESSED_TOTAL,
       help: 'Total number of payments processed',
       labelNames: ['status', 'method'],
     }),
     makeGaugeProvider({
-      name: 'active_subscriptions',
+      name: GOODGO_ACTIVE_SUBSCRIPTIONS,
       help: 'Number of active subscriptions',
       labelNames: ['plan'],
     }),
+
+    // ── Services & Interceptors ──
+    MetricsService,
+    HttpMetricsInterceptor,
   ],
-  exports: [PrometheusModule],
+  exports: [PrometheusModule, MetricsService, HttpMetricsInterceptor],
 })
 export class MetricsModule {}

apps/api/src/modules/metrics/presentation/interceptors/http-metrics.interceptor.ts

@@ -0,0 +1,47 @@
+import {
+  Injectable,
+  type CallHandler,
+  type ExecutionContext,
+  type NestInterceptor,
+} from '@nestjs/common';
+import { type Request, type Response } from 'express';
+import { type Observable, tap } from 'rxjs';
+// eslint-disable-next-line @typescript-eslint/consistent-type-imports
+import { MetricsService } from '../../infrastructure/metrics.service';
+
+@Injectable()
+export class HttpMetricsInterceptor implements NestInterceptor {
+  constructor(private readonly metricsService: MetricsService) {}
+
+  intercept(context: ExecutionContext, next: CallHandler): Observable<unknown> {
+    const httpContext = context.switchToHttp();
+    const request = httpContext.getRequest<Request>();
+    const startTime = process.hrtime.bigint();
+
+    return next.handle().pipe(
+      tap({
+        next: () => {
+          this.recordMetrics(request, httpContext.getResponse<Response>(), startTime);
+        },
+        error: () => {
+          this.recordMetrics(request, httpContext.getResponse<Response>(), startTime);
+        },
+      }),
+    );
+  }
+
+  private recordMetrics(
+    request: Request,
+    response: Response,
+    startTime: bigint,
+  ): void {
+    const durationNs = Number(process.hrtime.bigint() - startTime);
+    const durationSeconds = durationNs / 1e9;
+
+    const route = (request.route as { path?: string })?.path ?? request.path;
+    const method = request.method;
+    const statusCode = response.statusCode;
+
+    this.metricsService.recordHttpRequest(method, route, statusCode, durationSeconds);
+  }
+}