perf(http/prom): avoid oneshot channel for request duration start time

linkerd/http/prom/src/record_response.rs

@@ -81,7 +81,27 @@ where
 struct ResponseState<L: StreamLabel> {
     labeler: L,
     duration: DurationFamily<L::DurationLabels>,
-    start: oneshot::Receiver<time::Instant>,
+    start: StartTime,
+}
+
+/// Start time for a duration measurement.
+///
+/// Request duration knows the start time immediately whereas response duration
+/// needs to wait until the request body is fully flushed.
+pub(crate) enum StartTime {
+    /// Start time is already known.
+    Known(Option<time::Instant>),
+    /// Start time will be sent when the request body finishes streaming
+    Pending(oneshot::Receiver<time::Instant>),
+}
+
+impl StartTime {
+    fn recv(&mut self) -> Option<time::Instant> {
+        match self {
+            Self::Known(t) => t.take(),
+            Self::Pending(rx) => rx.try_recv().ok(),
+        }
+    }
 }
 
 type DurationFamily<L> = Family<L, Histogram, MkDurationHistogram>;
@@ -269,7 +289,7 @@ fn end_stream<L>(
 
     labeler.end_response(res);
 
-    let elapsed = if let Ok(start) = start.try_recv() {
+    let elapsed = if let Some(start) = start.recv() {
         time::Instant::now().saturating_duration_since(start)
     } else {
         time::Duration::ZERO

linkerd/http/prom/src/record_response/request.rs

@@ -8,7 +8,7 @@ use std::{
     sync::Arc,
     task::{Context, Poll},
 };
-use tokio::{sync::oneshot, time};
+use tokio::time;
 
 /// Metrics type that tracks completed requests.
 #[derive(Debug)]
@@ -81,8 +81,7 @@ where
 
     fn call(&mut self, req: http::Request<ReqB>) -> Self::Future {
         let state = self.labeler.mk_stream_labeler(&req).map(|labeler| {
-            let (tx, start) = oneshot::channel();
-            tx.send(time::Instant::now()).unwrap();
+            let start = super::StartTime::Known(Some(time::Instant::now()));
             let RequestMetrics { duration } = self.metric.clone();
             super::ResponseState {
                 labeler,

linkerd/http/prom/src/record_response/response.rs

@@ -95,7 +95,7 @@ where
             let ResponseMetrics { duration } = self.metric.clone();
             Some(super::ResponseState {
                 labeler,
-                start,
+                start: super::StartTime::Pending(start),
                 duration,
             })
         } else {