reverseproxy: Add duration/latency placeholders (close #4012) (#4013)

* reverseproxy: Add duration/latency placeholders (close #4012) (and #2268)

Adds 4 placeholders, one is actually outside reverse proxy though:

{http.request.duration} is how long since the server decoded the HTTP request (headers).
{http.reverse_proxy.upstream.latency} is how long it took a proxy upstream to write the response header.
{http.reverse_proxy.upstream.duration} is total time proxying to the upstream, including writing response body to client.
{http.reverse_proxy.duration} is total time spent proxying, including selecting an upstream and retries.

Obviously, most of these are only useful at the end of a request, like when writing response headers or logs.

See also: https://caddy.community/t/any-equivalent-of-request-time-and-upstream-header-time-from-nginx/11418

* Add new placeholders to documentation

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