metricsMiddleware: add comments

src/middleware/metricsMiddleware.ts

@@ -2,13 +2,19 @@ import { MiddlewareHandler } from '@hono/hono';
 
 import { httpRequestCounter, httpResponseCounter } from '@/metrics.ts';
 
+/** Prometheus metrics middleware that tracks HTTP requests by methods and responses by status code. */
 export const metricsMiddleware: MiddlewareHandler = async (c, next) => {
+  // HTTP Request.
   const { method } = c.req;
   httpRequestCounter.inc({ method });
 
+  // Wait for other handlers to run.
   await next();
 
+  // HTTP Response.
   const { status } = c.res;
+  // Get a parameterized path name like `/posts/:id` instead of `/posts/1234`.
+  // Tries to find actual route names first before falling back on potential middleware handlers like `app.use('*')`.
   const path = c.req.matchedRoutes.find((r) => r.method !== 'ALL')?.path ?? c.req.routePath;
   httpResponseCounter.inc({ status, path });
 };