Sanitize paths for Prometheus

[aulme]

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[aulme]

There already was some path sanitization attempt in code, but it didn't work. Fixed it as close to the original spirit/idea as I could. Also added a sprinkling of comments since not everything is obvious.
In absence of unit testing setup, testing done locally. Paths known to Express stay in metrics but are sanitized. The /user/:id comes from a test route I've added locally. Paths that are not known to Express including any spam get mapped to "unmatched-url".
Production tests for both UID2 and EUID work as per readme.

Unfortunately, any static files served get mapped to "unmatched-url" too since Express doesn't know about them. With a little effort it's possible to implement proper handling for them, but it wasn't in the original implementation either.