Extended options to allow custom path normalization

Author: Piotr Wyrobek

README.md

@@ -35,6 +35,9 @@ npm i --save express-prometheus-middleware
 | customLabels | Optional Array containing extra labels, used together with  `transformLabels` | no extra labels: `[]` |
 | transformLabels | Optional `function(labels, req, res)` adds to the labels object dynamic values for each label in `customLabels` | `null` |
 | normalizeStatus | Optional parameter to disable normalization of the status code. Example of normalized and non-normalized status code respectively: 4xx and 422.| true
+| customPathNormalizer | Optional `function(path, req, res)` to apply custom path normalization function | null
+
+
 ### Example
 
 ```js
@@ -73,6 +76,11 @@ app.use(promMid({
   //   // eslint-disable-next-line no-param-reassign
   //   labels.contentType = req.headers['content-type'];
   // },
+  // customPathNormalizer(path) {
+  //  if (path.includes('test')) {
+  //    reutrn 'test';
+  //  }
+  // }
 }));
 
 // curl -X GET localhost:9091/hello?name=Chuck%20Norris

src/index.js

@@ -25,6 +25,7 @@ const defaultOptions = {
   customLabels: [],
   transformLabels: null,
   normalizeStatus: true,
+  customPathNormalizer: null,
 };
 
 module.exports = (userOptions = {}) => {
@@ -56,7 +57,10 @@ module.exports = (userOptions = {}) => {
     // will replace ids from the route with `#val` placeholder this serves to
     // measure the same routes, e.g., /image/id1, and /image/id2, will be
     // treated as the same route
-    const route = normalizePath(originalUrl, options.extraMasks);
+    const customNormalizedPath = typeof option.customUrlNormalizer === 'function' ?
+      option.customPathNormalizer(originalUrl, req, res) : null;
+
+    const route = normalizePath(customNormalizedPath|| originalUrl, options.extraMasks);
 
     if (route !== metricsPath) {
       const status = normalizeStatus