ref(rust): Add join-time metric for threadpools (#362)

We suspect that joining a threadpool takes most of the time when
rebalancing.

rust-arroyo/src/processing/strategies/run_task_in_threads.rs

@@ -1,18 +1,18 @@
 use std::collections::VecDeque;
 use std::future::Future;
 use std::pin::Pin;
-use std::time::Duration;
+use std::time::{Duration, Instant};
 
 use tokio::runtime::{Handle, Runtime};
 use tokio::task::JoinHandle;
 
-use crate::gauge;
 use crate::processing::strategies::{
     merge_commit_request, CommitRequest, InvalidMessage, MessageRejected, ProcessingStrategy,
     SubmitError,
 };
 use crate::types::Message;
 use crate::utils::timing::Deadline;
+use crate::{gauge, timer};
 
 use super::StrategyError;
 
@@ -218,6 +218,7 @@ where
 
     fn join(&mut self, timeout: Option<Duration>) -> Result<Option<CommitRequest>, StrategyError> {
         let deadline = timeout.map(Deadline::new);
+        let start = Instant::now();
 
         // Poll until there are no more messages or timeout is hit
         while self.message_carried_over.is_some() || !self.handles.is_empty() {
@@ -240,6 +241,12 @@ where
         }
         self.handles.clear();
 
+        timer!(
+            "arroyo.strategies.run_task_in_threads.join_time",
+            start.elapsed(),
+            "strategy_name" => self.metric_strategy_name
+        );
+
         let next_commit = self.next_step.join(deadline.map(|d| d.remaining()))?;
 
         Ok(merge_commit_request(