Add prometheus exporter (#17392)

Create a metric endpoint and expose prometheus gauge for event stream
count.

packages/twenty-server/src/engine/subscriptions/event-stream.service.ts

@@ -1,4 +1,4 @@
-import { Injectable } from '@nestjs/common';
+import { Injectable, Logger, OnModuleInit } from '@nestjs/common';
 
 import { type RecordGqlOperationSignature } from 'twenty-shared/types';
 import { isDefined } from 'twenty-shared/utils';
@@ -9,6 +9,7 @@ import { WithLock } from 'src/engine/core-modules/cache-lock/with-lock.decorator
 import { InjectCacheStorage } from 'src/engine/core-modules/cache-storage/decorators/cache-storage.decorator';
 import { CacheStorageService } from 'src/engine/core-modules/cache-storage/services/cache-storage.service';
 import { CacheStorageNamespace } from 'src/engine/core-modules/cache-storage/types/cache-storage-namespace.enum';
+import { MetricsService } from 'src/engine/core-modules/metrics/metrics.service';
 import { EVENT_STREAM_TTL_MS } from 'src/engine/subscriptions/constants/event-stream-ttl.constant';
 import {
   EventStreamException,
@@ -17,13 +18,38 @@ import {
 import { type EventStreamData } from 'src/engine/subscriptions/types/event-stream-data.type';
 
 @Injectable()
-export class EventStreamService {
+export class EventStreamService implements OnModuleInit {
+  private readonly logger = new Logger(EventStreamService.name);
+
   constructor(
     @InjectCacheStorage(CacheStorageNamespace.EngineSubscriptions)
     private readonly cacheStorageService: CacheStorageService,
     private readonly cacheLockService: CacheLockService,
+    private readonly metricsService: MetricsService,
   ) {}
 
+  onModuleInit() {
+    this.metricsService.createObservableGauge(
+      'twenty_event_streams_live_total',
+      { description: 'Current number of live event streams' },
+      async (observableResult) => {
+        try {
+          const count = await this.getTotalActiveStreamCount();
+
+          observableResult.observe(count);
+        } catch (error) {
+          this.logger.error('Failed to collect event streams metrics', error);
+        }
+      },
+    );
+  }
+
+  async getTotalActiveStreamCount(): Promise<number> {
+    return this.cacheStorageService.scanAndCountSetMembers(
+      'workspace:*:activeStreams',
+    );
+  }
+
   async createEventStream({
     workspaceId,
     eventStreamChannelId,

packages/twenty-server/src/engine/subscriptions/subscriptions.module.ts

@@ -3,6 +3,7 @@ import { TypeOrmModule } from '@nestjs/typeorm';
 
 import { CacheLockModule } from 'src/engine/core-modules/cache-lock/cache-lock.module';
 import { CacheStorageModule } from 'src/engine/core-modules/cache-storage/cache-storage.module';
+import { MetricsModule } from 'src/engine/core-modules/metrics/metrics.module';
 import { RedisClientModule } from 'src/engine/core-modules/redis-client/redis-client.module';
 import { WorkspaceEntity } from 'src/engine/core-modules/workspace/workspace.entity';
 import { EventStreamService } from 'src/engine/subscriptions/event-stream.service';
@@ -13,6 +14,7 @@ import { SubscriptionService } from 'src/engine/subscriptions/subscription.servi
     RedisClientModule,
     CacheStorageModule,
     CacheLockModule,
+    MetricsModule,
     TypeOrmModule.forFeature([WorkspaceEntity]),
   ],
   providers: [SubscriptionService, EventStreamService],