refactor(http/prom): request::NewRecordBodyData is metrics agnostic

[cratelyn]

this commit makes a mechanical refactor in the interest of paving ground towards using the request body metrics middleware in the inbound proxy.

because of minor differences in how we represent protocols like gRPC in the inbound and outbound proxies, we must account for:

(a): distinct NewService<T>'s accepting Grpc<T> and Http<T> targets, respectively, each holding a single metrics Family<L>.

(b): a single NewService<T> that matches on the Permitted<T> enum to select one of two metrics Family<L>s.

see #4119 for more background on this Permitted<T> enum.

so, we should make the NewRecordBodyData agnostic to its metrics, rather than holding a concrete RequestBodyFamilies<L>.

this commit hoists the metrics out of the middleware, and into the X and ReqX extractors used in the NewService<T> and Service<T> layers, respectively.

bear particular attention to this part of the diff:

-    ReqX: ExtractParam<L, Request<ReqB>>,
-    L: linkerd_metrics::prom::encoding::EncodeLabelSet
-        + std::fmt::Debug
-        + std::hash::Hash
-        + Eq
-        + Clone
-        + Send
-        + Sync
-        + 'static,
+    ReqX: ExtractParam<BodyDataMetrics, Request<ReqB>>,

in other words, rather than using X and subsequently ReqX to extract our labels L, we expect these extractors to yield a BodyDataMetrics: one time series that should be incremented accordingly by the instrumented request.

in even simpler words: ReqX now calls familes.metrics(&labels) too.

[cratelyn]

i have rebased this atop main as of 0060a4c.

[cratelyn]

this branch has been rebased atop main, now that #4181 has landed.