Prometheus cleanup (#3583)

* Update prometheus metrics documentation

* Return 404 as default case for prometheus server

* Fix lint error

packages/client/README.md

@@ -308,7 +308,7 @@ dist/bin/cli.js --d
 ## Metrics
 
 The client can optionally collect metrics using the Prometheus metrics platform and expose them via an HTTP endpoint with the following CLI flags.  
-The current metrics that are reported by the client can be found [here](./src/util//metrics.ts).
+The current metrics that are reported by the client can be found at the default port and route: `localhost:8000/metrics`.
 
 ```sh
 # npm installation
@@ -318,8 +318,6 @@ ethereumjs --prometheus
 npm run client:start:ts -- --prometheus --prometheusPort=9123
 ```
 
-Note: The Prometheus endpoint runs on port 8000 by default
-
 ## API
 
 [API Reference](./docs/README.md)

packages/client/bin/cli.ts

@@ -1122,10 +1122,16 @@ async function run() {
       const reqUrl = new url.URL(req.url, `http://${req.headers.host}`)
       const route = reqUrl.pathname
 
-      if (route === '/metrics') {
-        // Return all metrics in the Prometheus exposition format
-        res.setHeader('Content-Type', register.contentType)
-        res.end(await register.metrics())
+      switch (route) {
+        case '/metrics':
+          // Return all metrics in the Prometheus exposition format
+          res.setHeader('Content-Type', register.contentType)
+          res.end(await register.metrics())
+          break
+        default:
+          res.statusCode = 404
+          res.end('Not found')
+          return
       }
     })
     // Start the HTTP server which exposes the metrics on http://localhost:${args.prometheusPort}/metrics