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

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 #3504. see the relevant component of that
  diff, here:
  <https://github.com/linkerd/linkerd2-proxy/pull/3504/files#diff-45d0bc344f76c111551a8eaf5d3f0e0c22ee6e6836a626e46402a6ae3cbc0035L262-R274>

* 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.

X-ref: #3504
X-ref: #3086
X-ref: linkerd/linkerd2#8733
Signed-off-by: katelyn martin <[email protected]>

Author: cratelyn

linkerd/http/prom/src/record_response.rs

@@ -268,6 +268,8 @@ where
             Some(Ok(frame)) => {
                 if let trls @ Some(_) = frame.trailers_ref() {
                     end_stream(this.state, Ok(trls));
+                } else if this.inner.is_end_stream() {
+                    end_stream(this.state, Ok(None));
                 }
             }
             Some(Err(error)) => end_stream(this.state, Err(error)),
@@ -278,7 +280,9 @@ where
     }
 
     fn is_end_stream(&self) -> bool {
-        self.inner.is_end_stream()
+        // If the inner response state is still in place, the end of the stream has not been
+        // classified and recorded yet.
+        self.state.is_none()
     }
 }