Support ignoring request paths with regular-expression patterns

[rovazh]

Current Situation

When an application uses the standard net/http mux, every request whose path is not listed in IgnoredPaths is recorded.
Servers are frequently hit by automated scanners, health-checkers, or other unpredictable clients. These one-off paths continuously create new Prometheus time-series, causing unbounded cardinality growth.

Problem

Because the paths are effectively random (e.g. /.git/config, /favicon_123.ico, …), it is not feasible to maintain an exhaustive IgnoredPaths list.

Proposed Solution

Add a new configuration field to the middleware factory:

// IgnoredPathRegexps is a list of regular expression path patterns that will not be
// measured for the request duration and the response size. They will still be counted
// in the RequestsInflight metric.
IgnoredPathRegexps []string

[rovazh]

Hello @slok! Please take a look.
Thank you!

[deltamualpha]

I also recently ran into a case where a pentest scanned across hundreds of thousands of paths in my application. all of them 404, but resulting in a vast explosion of garbage cardinality. Left unchecked, this can actually cause significant memory usage, and could be a denial-of-service vector. Would you accept a PR for this functionality @slok?

It might even make more sense to have an allowlist as opposed to a denylist here...