feat(observability): add in-process metrics endpoint for qps latency and error-rate

Fabric.Backend.Center/src/app.module.ts

@@ -2,6 +2,8 @@ import { Module } from '@nestjs/common';
 import { TypeOrmModule } from '@nestjs/typeorm';
 import { buildTypeOrmConfig } from './database.config';
 import { HealthController } from './common/health.controller';
+import { MetricsController } from './common/metrics.controller';
+import { MetricsService } from './common/metrics.service';
 import { AuthModule } from './auth/auth.module';
 import { NodesModule } from './nodes/nodes.module';
 import { AuditModule } from './audit/audit.module';
@@ -13,6 +15,7 @@ import { AuditModule } from './audit/audit.module';
     AuthModule,
     NodesModule,
   ],
-  controllers: [HealthController],
+  controllers: [HealthController, MetricsController],
+  providers: [MetricsService],
 })
 export class AppModule {}

Fabric.Backend.Center/src/common/metrics.controller.ts

@@ -0,0 +1,12 @@
+import { Controller, Get } from '@nestjs/common';
+import { MetricsService } from './metrics.service';
+
+@Controller('metrics')
+export class MetricsController {
+  constructor(private readonly metrics: MetricsService) {}
+
+  @Get()
+  get() {
+    return this.metrics.snapshot();
+  }
+}

Fabric.Backend.Center/src/common/metrics.service.ts

@@ -0,0 +1,35 @@
+import { Injectable } from '@nestjs/common';
+
+type Bucket = {
+  requests: number;
+  errors: number;
+  totalDurationMs: number;
+};
+
+@Injectable()
+export class MetricsService {
+  private readonly bucket: Bucket = { requests: 0, errors: 0, totalDurationMs: 0 };
+  private startedAt = Date.now();
+
+  record(statusCode: number, durationMs: number): void {
+    this.bucket.requests += 1;
+    if (statusCode >= 400) this.bucket.errors += 1;
+    this.bucket.totalDurationMs += durationMs;
+  }
+
+  snapshot() {
+    const uptimeSec = Math.max(1, Math.floor((Date.now() - this.startedAt) / 1000));
+    const qps = this.bucket.requests / uptimeSec;
+    const avgLatencyMs = this.bucket.requests > 0 ? this.bucket.totalDurationMs / this.bucket.requests : 0;
+    const errorRate = this.bucket.requests > 0 ? this.bucket.errors / this.bucket.requests : 0;
+
+    return {
+      requests: this.bucket.requests,
+      errors: this.bucket.errors,
+      qps,
+      avgLatencyMs,
+      errorRate,
+      uptimeSec,
+    };
+  }
+}

Fabric.Backend.Center/src/common/request-context.middleware.ts

@@ -1,30 +1,36 @@
 import { randomUUID } from 'crypto';
 import { NextFunction, Request, Response } from 'express';
+import { MetricsService } from './metrics.service';
 
 type ReqWithId = Request & { requestId?: string };
 
-export function requestContextMiddleware(req: ReqWithId, res: Response, next: NextFunction): void {
-  const headerId = req.headers['x-request-id'];
-  const requestId =
-    (Array.isArray(headerId) ? headerId[0] : headerId) || randomUUID();
+export function createRequestContextMiddleware(service: 'center' | 'guild', metrics: MetricsService) {
+  return (req: ReqWithId, res: Response, next: NextFunction): void => {
+    const headerId = req.headers['x-request-id'];
+    const requestId =
+      (Array.isArray(headerId) ? headerId[0] : headerId) || randomUUID();
 
-  req.requestId = requestId;
-  res.setHeader('x-request-id', requestId);
+    req.requestId = requestId;
+    res.setHeader('x-request-id', requestId);
 
-  const startedAt = Date.now();
-  res.on('finish', () => {
-    const log = {
-      level: 'info',
-      service: 'center',
-      requestId,
-      method: req.method,
-      path: req.originalUrl,
-      statusCode: res.statusCode,
-      durationMs: Date.now() - startedAt,
-      timestamp: new Date().toISOString(),
-    };
-    console.log(JSON.stringify(log));
-  });
+    const startedAt = Date.now();
+    res.on('finish', () => {
+      const durationMs = Date.now() - startedAt;
+      metrics.record(res.statusCode, durationMs);
 
-  next();
+      const log = {
+        level: 'info',
+        service,
+        requestId,
+        method: req.method,
+        path: req.originalUrl,
+        statusCode: res.statusCode,
+        durationMs,
+        timestamp: new Date().toISOString(),
+      };
+      console.log(JSON.stringify(log));
+    });
+
+    next();
+  };
 }

Fabric.Backend.Center/src/main.ts

@@ -3,7 +3,8 @@ import { ValidationPipe } from '@nestjs/common';
 import { NestFactory } from '@nestjs/core';
 import { DocumentBuilder, SwaggerModule } from '@nestjs/swagger';
 import { AppModule } from './app.module';
-import { requestContextMiddleware } from './common/request-context.middleware';
+import { createRequestContextMiddleware } from './common/request-context.middleware';
+import { MetricsService } from './common/metrics.service';
 
 function requireEnv(name: string): string {
   const value = process.env[name];
@@ -29,7 +30,8 @@ async function bootstrap() {
 
   const app = await NestFactory.create(AppModule);
   app.setGlobalPrefix('api');
-  app.use(requestContextMiddleware);
+  const metrics = app.get(MetricsService);
+  app.use(createRequestContextMiddleware('center', metrics));
   app.useGlobalPipes(
     new ValidationPipe({
       whitelist: true,