refactor(app/inbound): type aliases specify extractors

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.

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::{NewCountRequests, RequestCount},
+    count_reqs::{self, RequestCount},
 };
 
 pub(super) fn layer<N>(
@@ -22,20 +22,7 @@ pub(super) fn layer<N>(
         response_body_data,
         ..
     }: &InboundMetrics,
-) -> impl svc::Layer<
-    N,
-    Service = NewCountRequests<
-        ExtractRequestCount,
-        body_data::response::NewRecordBodyData<
-            ExtractResponseBodyDataMetrics,
-            body_data::request::NewRecordBodyData<
-                N,
-                ExtractRequestBodyDataParams,
-                ExtractRequestBodyDataMetrics,
-            >,
-        >,
-    >,
-> {
+) -> impl svc::Layer<N, Service = Instrumented<N>> {
     use svc::Layer as _;
 
     let count = {
@@ -45,17 +32,23 @@ 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)
     };
 
     let request = {
         let extract = ExtractRequestBodyDataParams(request_body_data.clone());
-        body_data::request::NewRecordBodyData::new(extract)
+        NewRecordRequestBodyData::new(extract)
     };
 
     svc::layer::mk(move |inner| count.layer(body.layer(request.layer(inner))))
 }
 
+/// An `N`-typed service instrumented with metrics middleware.
+type Instrumented<N> = NewCountRequests<NewRecordResponseBodyData<NewRecordRequestBodyData<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>,
@@ -70,6 +63,10 @@ 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>,
@@ -84,6 +81,13 @@ pub struct ResponseBodyDataLabels {
 #[derive(Clone, Debug)]
 pub struct ExtractResponseBodyDataMetrics(ResponseBodyFamilies);
 
+/// An `N`-typed `NewService<T>` instrumented with request body metrics.
+type NewRecordRequestBodyData<N> = body_data::request::NewRecordBodyData<
+    N,
+    ExtractRequestBodyDataParams,
+    ExtractRequestBodyDataMetrics,
+>;
+
 #[derive(Clone, Debug)]
 pub struct RequestBodyFamilies {
     grpc: body_data::request::RequestBodyFamilies<RequestBodyDataLabels>,