Add per route metrics option

[rtreffer-rddt]

This is a continuation of #5535

I made sure I keep all signed-off and author attributions intact, I added a signed-off when I changed things. The rebase required mostly a change from ref.To to ptr.To.

There is one significant addition. HTTPProxy objects can define multiple routes, so we would need a way to distinguish routes. For that I'll per default generate a router+index suffix (route0, route1, ...). There is optionally also support for a RouteTag that can be used for semantic names.

Fixes #4637