refactor(app/inbound): metrics layer accepts InboundMetrics (#4166)

this commit makes a small refactor to the inbound proxy's
`metrics::layer()` middleware in the interest of future-proofing it
before additional metrics layers are introduced.

this will mean that the call site in `http/router.rs` will not need to
be updated repeatedly as we introduce request/response body frame size
metrics, duration histograms, and so forth.

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

Author: cratelyn

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

@@ -245,7 +245,7 @@ impl<C> Inbound<C> {
                 .push(svc::NewOneshotRoute::layer_via(|t: &policy::Permitted<T>| {
                     LogicalPerRequest::from(t)
                 }))
-                .push(self::metrics::layer(rt.metrics.request_count.clone()))
+                .push(self::metrics::layer(&rt.metrics))
                 .check_new_service::<policy::Permitted<T>, http::Request<http::BoxBody>>()
                 .push(svc::ArcNewService::layer())
                 .push(policy::NewHttpPolicy::layer(rt.metrics.http_authz.clone()))

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

@@ -1,4 +1,4 @@
-use crate::policy::Permitted;
+use crate::{policy::Permitted, InboundMetrics};
 use linkerd_app_core::{
     metrics::{
         prom::{
@@ -13,14 +13,16 @@ use linkerd_app_core::{
 use linkerd_http_prom::count_reqs::{NewCountRequests, RequestCount};
 
 pub(super) fn layer<N>(
-    request_count: RequestCountFamilies,
-    // TODO(kate): other metrics families will added here.
+    InboundMetrics { request_count, .. }: &InboundMetrics,
 ) -> impl svc::Layer<N, Service = NewCountRequests<ExtractRequestCount, N>> {
-    svc::layer::mk(move |inner| {
-        use svc::Layer as _;
+    use svc::Layer as _;
+
+    let count = {
         let extract = ExtractRequestCount(request_count.clone());
-        NewCountRequests::layer_via(extract).layer(inner)
-    })
+        NewCountRequests::layer_via(extract)
+    };
+
+    svc::layer::mk(move |inner| count.layer(inner))
 }
 
 #[derive(Clone, Debug)]