refactor(app): Outline route label extractor (#3337)

* refactor(app): Hoist label extractor out of retry middleware

this commit promotes the `RetryLabelExtract` type out of the route retry
middleware, and into the metrics submodule.

in preparation for generalizing this component, we rename it to
`RouteLabelExtract`.

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

* refactor(app): Outline route label extraction

this addresses the `TODO` comment, moving the `RouteLabelExtract` type
out of the policy route layer function.

this defines a method on `MatchedRoute` which may be called to retrieve
a `RouteLabelExtract`. that type holds references to the
contruction-time information about the route, and can be used to later
inspect an outbound request at call-time, returning a `labels::Route`
set of labels.

this is a helpful piece of reusable glue that is broadly useful in
instrumenting our route-level middleware.

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

---------

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

linkerd/app/outbound/src/http/logical/policy/route.rs

@@ -124,18 +124,9 @@ where
                 .push_on_service(svc::LoadShed::layer())
                 .push(filters::NewApplyFilters::<Self, _, _>::layer())
                 .push({
-                    // TODO(kate): extracting route labels like this should ideally live somewhere
-                    // else, like e.g. the `SetExtensions` middleware.
-                    let mk_extract = |rt: &Self| {
-                        let Route {
-                            parent_ref,
-                            route_ref,
-                            ..
-                        } = &rt.params;
-                        retry::RetryLabelExtract(parent_ref.clone(), route_ref.clone())
-                    };
+                    let mk = Self::label_extractor;
                     let metrics = metrics.retry.clone();
-                    retry::NewHttpRetry::layer_via_mk(mk_extract, metrics)
+                    retry::NewHttpRetry::layer_via_mk(mk, metrics)
                 })
                 .check_new::<Self>()
                 .check_new_service::<Self, http::Request<http::BoxBody>>()

linkerd/app/outbound/src/http/logical/policy/route/metrics/labels.rs

@@ -1,6 +1,7 @@
 //! Prometheus label types.
 use linkerd_app_core::{
-    dns, errors, metrics::prom::EncodeLabelSetMut, proxy::http, Error as BoxError,
+    dns, errors, metrics::prom::EncodeLabelSetMut, proxy::http, svc::ExtractParam,
+    Error as BoxError,
 };
 use prometheus_client::encoding::*;
 
@@ -39,6 +40,9 @@ pub struct GrpcRsp {
     pub error: Option<Error>,
 }
 
+#[derive(Clone, Debug)]
+pub struct RouteLabelExtract(pub ParentRef, pub RouteRef);
+
 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
 pub enum Error {
     FailFast,
@@ -211,6 +215,37 @@ impl EncodeLabelSet for GrpcRsp {
     }
 }
 
+// === impl MatchedRoute ===
+
+impl<T, M, F, P> super::super::MatchedRoute<T, M, F, P> {
+    /// Returns a [`RouteLabelExtract`].
+    ///
+    /// The extractor returned by this function provides a [`ExtractParam<P, T>`] implementation
+    /// that can be used to acquire the route-level labels corresponding to a given outbound
+    /// request.
+    pub(crate) fn label_extractor(&self) -> RouteLabelExtract {
+        use super::super::Route;
+        let Route {
+            parent_ref,
+            route_ref,
+            ..
+        } = &self.params;
+
+        RouteLabelExtract(parent_ref.clone(), route_ref.clone())
+    }
+}
+
+// === impl RouteLabelExtract ===
+
+impl<B> ExtractParam<Route, http::Request<B>> for RouteLabelExtract {
+    fn extract_param(&self, t: &http::Request<B>) -> Route {
+        let Self(parent, route) = self;
+        let uri = t.uri();
+
+        Route::new(parent.clone(), route.clone(), uri)
+    }
+}
+
 // === impl Error ===
 
 impl Error {

linkerd/app/outbound/src/http/logical/policy/route/retry.rs

@@ -1,12 +1,14 @@
-use super::{extensions, metrics::labels::Route as RouteLabels};
-use crate::{ParentRef, RouteRef};
+use super::{
+    extensions,
+    metrics::labels::{Route as RouteLabels, RouteLabelExtract},
+};
 use futures::future::{Either, Ready};
 use linkerd_app_core::{
     cause_ref, classify,
     exp_backoff::ExponentialBackoff,
     is_caused_by,
     proxy::http::{self, stream_timeouts::ResponseTimeoutError},
-    svc::{self, http::h2, ExtractParam},
+    svc::{self, http::h2},
     Error, Result,
 };
 use linkerd_http_retry::{self as retry, peek_trailers::PeekTrailersBody};
@@ -18,7 +20,7 @@ use tokio::time;
 pub struct IsRetry(());
 
 pub type NewHttpRetry<F, N> =
-    retry::NewHttpRetry<RetryPolicy, RouteLabels, F, RetryLabelExtract, N>;
+    retry::NewHttpRetry<RetryPolicy, RouteLabels, F, RouteLabelExtract, N>;
 
 #[derive(Clone, Debug)]
 pub struct RetryPolicy {
@@ -31,9 +33,6 @@ pub struct RetryPolicy {
     pub backoff: Option<ExponentialBackoff>,
 }
 
-#[derive(Clone, Debug)]
-pub struct RetryLabelExtract(pub ParentRef, pub RouteRef);
-
 pub type RouteRetryMetrics = retry::MetricFamilies<RouteLabels>;
 
 // === impl RetryPolicy ===
@@ -163,14 +162,3 @@ impl RetryPolicy {
         false
     }
 }
-
-// === impl RetryLabelExtract ===
-
-impl<B> ExtractParam<RouteLabels, http::Request<B>> for RetryLabelExtract {
-    fn extract_param(&self, t: &http::Request<B>) -> RouteLabels {
-        let Self(parent, route) = self;
-        let uri = t.uri();
-
-        RouteLabels::new(parent.clone(), route.clone(), uri)
-    }
-}