feat(api): add prometheus metrics

Currently we support metrics for the Node.js runtime and HTTP endpoints.

src/app.module.ts

@@ -9,6 +9,7 @@ import { DatabaseModule } from "./database/database.module";
 import { HealthCheckModule } from "./health-check/health-check.module";
 import { ListensModule } from "./listens/listens.module";
 import { LoggerModule } from "./logger/logger.module";
+import { MetricsModule } from "./metrics/metrics.module";
 import { MusicLibraryModule } from "./music-library/music-library.module";
 import { ReportsModule } from "./reports/reports.module";
 import { SourcesModule } from "./sources/sources.module";
@@ -25,6 +26,7 @@ import { UsersModule } from "./users/users.module";
       exclude: ["/api*"],
     }),
     RavenModule,
+    MetricsModule.forRoot(),
     AuthModule,
     UsersModule,
     SourcesModule,

src/config/config.module.ts

@@ -42,6 +42,9 @@ import { ConfigModule as NestConfigModule } from "@nestjs/config";
           is: Joi.valid(true),
           then: Joi.required(),
         }),
+
+        // Prometheus for Metrics (Optional)
+        PROMETHEUS_ENABLED: Joi.boolean().default(false),
       }),
     }),
   ],

src/metrics/metrics.module.ts

@@ -0,0 +1,51 @@
+import { InboundMiddleware, PromModule } from "@digikare/nestjs-prom";
+import { DEFAULT_PROM_OPTIONS } from "@digikare/nestjs-prom/dist/prom.constants";
+import {
+  DynamicModule,
+  MiddlewareConsumer,
+  Module,
+  NestModule,
+} from "@nestjs/common";
+
+// Dirty hack because we can not conditionally import modules based on
+// injected services and upstream module does not support dynamic configuration
+//
+// https://github.com/digikare/nestjs-prom/issues/27
+const promEnabled = process.env.PROMETHEUS_ENABLED === "true";
+
+@Module({})
+export class MetricsModule implements NestModule {
+  static forRoot(): DynamicModule {
+    const module = {
+      imports: [],
+      providers: [],
+    };
+    if (promEnabled) {
+      const promOptions = {
+        metricPath: "/api/metrics",
+        withDefaultsMetrics: true,
+        withDefaultController: true,
+      };
+
+      module.imports.push(PromModule.forRoot(promOptions));
+
+      module.providers.push({
+        provide: DEFAULT_PROM_OPTIONS,
+        useValue: promOptions,
+      });
+    }
+
+    return {
+      module: MetricsModule,
+      ...module,
+    };
+  }
+
+  configure(consumer: MiddlewareConsumer) {
+    if (promEnabled) {
+      // We register the Middleware ourselves to avoid tracking
+      // latency for static files served for the frontend.
+      consumer.apply(InboundMiddleware).forRoutes("/api");
+    }
+  }
+}