refactor(app/inbound): router metrics middleware are T-agnostic

[cratelyn]

#4119 introduced a Permitted<T> structure, to provide a more formal notion of a (HttpRoutePermit, T) tuple.

during review of #4180, we had a discussion about how the inbound proxy's http router had metrics layers which were tightly coupled to the notion of a Permitted<T>:

This isn't a hard blocker--but this is unfortunate that we're coupling this impl to the Permitted target type -- target types are usually an internal implementation detail and not a public api that is depended on elsewhere.

The typical pattern for this would be to have a dump bespoke param in this module like:

pub enum MetricsVariant { Http, Grpc }

and then have our Permitted impl Param<MetricsVariant> for Permitted<T>.

then this module remains decoupled from the concrete target types.

That is, the metrics module is decoupled from where it's called--as long as the caller provides a target that can give us what we need to build our extractors, we're good to go.

this branch performs a series of changes to explore what appears to be a promising alternative to the existing model of Permitted<T>. importantly, this has the effect of leaving the inbound metrics layers' extractors agnostic to their target: they now provide implementations like svc::ExtractParam<BodyDataMetrics, T> rather than svc::ExtractParam<BodyDataMetrics, Permitted<T>>.

• feat(app/inbound): request body frame size metrics #4180