fix(http/prom): record bodies when eos reached

[cratelyn]

this commit fixes a bug discovered by @alpeb, which was introduced in proxy
v2.288.0.

The associated metric is outbound_http_route_request_statuses_total:

$ linkerd dg proxy-metrics -n booksapp deploy/webapp|rg outbound_http_route_request_statuses_total.*authors
outbound_http_route_request_statuses_total{parent_group="core",parent_kind="Service",parent_namespace="booksapp",parent_name="authors",parent_port="7001",parent_section_name="",route_group="",route_kind="default",route_namespace="",route_name="http",hostname="",http_status="204",error=""} 5
outbound_http_route_request_statuses_total{parent_group="core",parent_kind="Service",parent_namespace="booksapp",parent_name="authors",parent_port="7001",parent_section_name="",route_group="",route_kind="default",route_namespace="",route_name="http",hostname="",http_status="201",error="UNKNOWN"} 5
outbound_http_route_request_statuses_total{parent_group="core",parent_kind="Service",parent_namespace="booksapp",parent_name="authors",parent_port="7001",parent_section_name="",route_group="",route_kind="default",route_namespace="",route_name="http",hostname="",http_status="200",error="UNKNOWN"} 10

The problem was introduced in edge-25.3.4, with the proxy v2.288.0.
Before that the metrics looked like:

$ linkerd dg proxy-metrics -n booksapp deploy/webapp|rg outbound_http_route_request_statuses_total.*authors
outbound_http_route_request_statuses_total{parent_group="core",parent_kind="Service",parent_namespace="booksapp",parent_name="authors",parent_port="7001",parent_section_name="",route_group="",route_kind="default",route_namespace="",route_name="http",hostname="",http_status="200",error=""} 193
outbound_http_route_request_statuses_total{parent_group="core",parent_kind="Service",parent_namespace="booksapp",parent_name="authors",parent_port="7001",parent_section_name="",route_group="",route_kind="default",route_namespace="",route_name="http",hostname="",http_status="204",error=""} 96
outbound_http_route_request_statuses_total{parent_group="core",parent_kind="Service",parent_namespace="booksapp",parent_name="authors",parent_port="7001",parent_section_name="",route_group="",route_kind="default",route_namespace="",route_name="http",hostname="",http_status="201",error=""} 96

So the difference is the non-empty value for error=UNKNOWN even
when https_status is 2xx, which linkerd viz stat-outbound
interprets as failed requests.

in #3086 we introduced a suite of route- and backend-level metrics. that
subsystem contains a body middleware that will report itself as having
reached the end-of-stream by delegating directly down to its inner
body's is_end_stream() hint.

this is roughly correct, but is slightly distinct from the actual invariant: a
linkerd_http_prom::record_response::ResponseBody<B> must call its
end_stream helper to classify the outcome and increment the corresponding
time series in the outbound_http_route_request_statuses_total metric family.

in #3504 we upgraded our hyper dependency. while doing so, we neglected to
include a call to end_stream if a data frame is yielded and the inner body
reports itself as having reached the end-of-stream.

this meant that instrumented bodies would be polled until the end is reached,
but were being dropped before a None was encountered.

this commit fixes this issue in two ways, to be defensive:

• invoke end_stream() if a non-trailers frame is yielded, and the inner body
  now reports itself as having ended. this restores the behavior in place prior
  to chore(deps)!: upgrade to hyper 1.x #3504. see the relevant component of that diff, here:
  https://github.com/linkerd/linkerd2-proxy/pull/3504/files#diff-45d0bc344f76c111551a8eaf5d3f0e0c22ee6e6836a626e46402a6ae3cbc0035L262-R274

-        let res =
-            futures::ready!(this.inner.as_mut().poll_data(cx)).map(|res| res.map_err(Into::into));
-        if let Some(Err(error)) = res.as_ref() {
-            end_stream(this.state, Err(error));
-        } else if (*this.inner).is_end_stream() {
-            end_stream(this.state, Ok(None));
+
+        // Poll the inner body for the next frame.
+        let poll = this.inner.as_mut().poll_frame(cx);
+        let frame = futures::ready!(poll).map(|res| res.map_err(Error::from));
+
+        match &frame {
+            Some(Ok(frame)) => {
+                if let trls @ Some(_) = frame.trailers_ref() {
+                    end_stream(this.state, Ok(trls));
+                }
+            }
+            Some(Err(error)) => end_stream(this.state, Err(error)),
+            None => end_stream(this.state, Ok(None)),

• rather than delegating to the inner <B as Body>::is_end_stream() method,
  report the end-of-stream being reached by inspecting whether or not the inner
  response state has been taken. this is the state that directly indicates
  whether or not the ResponseBody<B> middleware is finished.

this adds a linkerd-mock-http-body development dependency, so we can use this
mock body type in the outbound proxy's unit tests. additional test coverage for
http and grpc routes' metrics is added to demonstrate that this fix works as
expected.

X-ref: #3504
X-ref: #3086
X-ref: linkerd/linkerd2#8733