chore(http/prom): body_data exports BodyDataMetrics

this structure representing the frame size metrics is shared by the
request and response instrumentation.

the `body_data::request` and `body_data::response` submodules reëxport
this, because the outbound proxy which uses these middleware separately
in route-level and backend-level metrics layers. the inbound proxy,
however, uses them each in the same place (see
#4180), which is ergonomically odd.

this commit introduces a top-level `pub use`, exposing this metrics
structure from `body_data`, for consumers that instrument request and
response bodies in one place.

Signed-off-by: katelyn martin <[email protected]>

Author: cratelyn

linkerd/http/prom/src/body_data.rs

@@ -1,3 +1,5 @@
+pub use self::metrics::BodyDataMetrics;
+
 pub mod request;
 pub mod response;