* Adds custom dial func for HTTP-Connect and socks5 proxy tunnels.
Standard golang http.transport exposes GetProxyConnectHeader function,
but it doesn't allow to use separate tls config for proxy.
It also not possible to enforce HTTP-Connect with standard http lib.
* For http scrape targets, by default http.Transport.Proxy function must
be used. Since it has special case with full uri forward.
* Adds proxy.URL json methods that allow to properly copy internal
fields, like User/Password.
It should fix bug with proxy_url. When credentials specified at URL was
ignored.
* Adds tests for scrape client proxy requests
related issue https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6771
- Rename GetStatDialFunc to NewStatDialFunc, since it returns new function with every call
- NewStatDialFunc isn't related to http in any way, so it must be moved from lib/httputils to lib/netutil
- Simplify the implementation of NewStatDialFunc by removing sync.Map from there.
- Use netutil.NewStatDialFunc at app/vmauth and lib/promscrape/discoveryutils
- Use gauge instead of counter type for *_conns metric
This is a follow-up for d7b5062917
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6299
