fix: ensure final message is always delivered (#4569)

crates/metrics/src/common/mpsc.rs

@@ -11,6 +11,7 @@ use std::{
 use tokio::sync::mpsc::{
     self,
     error::{SendError, TryRecvError, TrySendError},
+    OwnedPermit,
 };
 
 /// Wrapper around [mpsc::unbounded_channel] that returns a new unbounded metered channel.
@@ -142,6 +143,18 @@ impl<T> MeteredSender<T> {
         Self { sender, metrics: MeteredSenderMetrics::new(scope) }
     }
 
+    /// Tries to acquire a permit to send a message.
+    ///
+    /// See also [Sender](mpsc::Sender)'s `try_reserve_owned`.
+    pub fn try_reserve_owned(&self) -> Result<OwnedPermit<T>, TrySendError<mpsc::Sender<T>>> {
+        self.sender.clone().try_reserve_owned()
+    }
+
+    /// Returns the underlying [Sender](mpsc::Sender).
+    pub fn inner(&self) -> &mpsc::Sender<T> {
+        &self.sender
+    }
+
     /// Calls the underlying [Sender](mpsc::Sender)'s `try_send`, incrementing the appropriate
     /// metrics depending on the result.
     pub fn try_send(&self, message: T) -> Result<(), TrySendError<T>> {