Exclude /metrics and /status routes from session initialization

This commit adds a `useUnless` helper method which can be used as a middleware for express.
It receives an express-middleware and an array of paths.
When a request matches one of the given paths, this middleware does nothing.
Otherwise the given middleware is called.

For the express-session middleware this helper middleware is used to avoid session creation on purely status routes.
See #1446

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

lib/utils.js

@@ -25,3 +25,12 @@ exports.getImageMimeType = function getImageMimeType (imagePath) {
       return undefined
   }
 }
+
+exports.useUnless = function excludeRoute (paths, middleware) {
+  return function (req, res, next) {
+    if (paths.includes(req.path)) {
+      return next()
+    }
+    return middleware(req, res, next)
+  }
+}