refactor(app/inbound): type aliases specify extractors (#4189)

our metrics layer is generic across an `N`-typed service factory.

this commit introduces types aliases that apply `X`-typed extractor
parameters to the `linkerd-http-prom` middleware, which are unaware of
a concrete metrics extractor.

we use this to reduce the complexity of the `layer()` function's
type signature.

see #4180.

NB: based upon #4188, and
#4186.

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

Author: cratelyn

linkerd/app/inbound/src/http/router/metrics.rs

@@ -12,7 +12,7 @@ use linkerd_app_core::{
 };
 use linkerd_http_prom::{
     body_data::{self, BodyDataMetrics},
-    count_reqs::{self, NewCountRequests, RequestCount},
+    count_reqs::{self, RequestCount},
 };
 
 pub(super) fn layer<N>(
@@ -21,13 +21,7 @@ pub(super) fn layer<N>(
         response_body_data,
         ..
     }: &InboundMetrics,
-) -> impl svc::Layer<
-    N,
-    Service = NewCountRequests<
-        ExtractRequestCount,
-        body_data::response::NewRecordBodyData<ExtractResponseBodyDataMetrics, N>,
-    >,
-> {
+) -> impl svc::Layer<N, Service = Instrumented<N>> {
     use svc::Layer as _;
 
     let count = {
@@ -37,12 +31,18 @@ pub(super) fn layer<N>(
 
     let body = {
         let extract = ExtractResponseBodyDataMetrics(response_body_data.clone());
-        body_data::response::NewRecordBodyData::layer_via(extract)
+        NewRecordResponseBodyData::layer_via(extract)
     };
 
     svc::layer::mk(move |inner| count.layer(body.layer(inner)))
 }
 
+/// An `N`-typed service instrumented with metrics middleware.
+type Instrumented<N> = NewCountRequests<NewRecordResponseBodyData<N>>;
+
+/// An `N`-typed `NewService<T>` instrumented with request counting metrics.
+type NewCountRequests<N> = count_reqs::NewCountRequests<ExtractRequestCount, N>;
+
 #[derive(Clone, Debug)]
 pub struct RequestCountFamilies {
     grpc: count_reqs::RequestCountFamilies<RequestCountLabels>,
@@ -57,6 +57,10 @@ pub struct RequestCountLabels {
 #[derive(Clone, Debug)]
 pub struct ExtractRequestCount(pub RequestCountFamilies);
 
+/// An `N`-typed `NewService<T>` instrumented with response body metrics.
+type NewRecordResponseBodyData<N> =
+    body_data::response::NewRecordBodyData<ExtractResponseBodyDataMetrics, N>;
+
 #[derive(Clone, Debug)]
 pub struct ResponseBodyFamilies {
     grpc: body_data::response::ResponseBodyFamilies<ResponseBodyDataLabels>,