Default API metrics for NestJS with fastify records full url path instead of full route

[hesham-desouky]

NestJS version: 8.0.0
nestjs-otel version: 3.0.1

NestJS-otel initialization code

    OpenTelemetryModule.forRoot({
      metrics: {
        hostMetrics: true, // Includes Host Metrics
        defaultMetrics: true, // Includes Default Metrics
        apiMetrics: {
          enable: true, // Includes api metrics
          timeBuckets: [], // You can change the default time buckets
          ignoreRoutes: ['/favicon.ico'], // You can ignore specific routes (See https://docs.nestjs.com/middleware#excluding-routes for options)
          ignoreUndefinedRoutes: true, //Records metrics for all URLs, even undefined ones
        },
      },
    })

NestJS App Initialization code

  const app = await NestFactory.create<NestFastifyApplication>(
    AppModule,
    new FastifyAdapter({ logger: false }),
  );

The default metric API collects the full URL instead of the route URL

example
/api/user/1 will be reported as /api/user/1 instead of /api/user/:id

The metric endpoint generate thousands of metrics for each id parameters instead of 1 metric only for this specific route

When i debugged the code, i found out that the default metric middle ware can't retrieve the route object from the request object.

[fabiozig]

any plans for this to be fixed?

[fabiozig]

I did some research on this, and apparently, the issue is that fastify uses middie to support middlewares, and instead of using the express request object, it uses the req.raw value. That's why the req.route value in the code below is undefined when using fastify.
https://github.com/pragmaticivan/nestjs-otel/blob/main/src/middleware/api-metrics.middleware.ts#L99

here is an explanation of the middie/fastify/middleware problem
nestjs/nest#9927 (comment)

It seems like if we use interceptors instead we would be able to get the fastify request and then the route we want...

const fastifyRequest = context.switchToHttp().getRequest()
fastifyRequest.routerPath...

[ttshivers]

I worked around this by adding route.path to the fastify raw request with an interceptor.

import type { CallHandler, ExecutionContext, NestInterceptor } from '@nestjs/common';
import { Injectable } from '@nestjs/common';
import type { FastifyRequest, RawRequestDefaultExpression } from 'fastify';
import type { Observable } from 'rxjs';

interface RequestRoute {
  route: {
    path: string;
  };
}

/**
 * Add the router path so it is visibile in the middleware as req.route.path which
 * nestjs-otel uses for route metrics.
 * See https://github.com/pragmaticivan/nestjs-otel/issues/245
 */
@Injectable()
export class FastifyRoutePathInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<unknown> {
    const request = context.switchToHttp().getRequest<FastifyRequest>();

    // eslint-disable-next-line no-type-assertion/no-type-assertion
    const raw = request.raw as RawRequestDefaultExpression & RequestRoute;
    raw.route = {
      path: request.routeOptions.url,
    };

    return next.handle();
  }
}