chore: Add a timer to avoid repeated debug logs (#954)

## Problem
When the Lambda runtime spins down, the extension may enter a loop
waiting for unfinished work, printing up to 100,000s identical lines of
log:
> LOGS_AGENT | No more events to process but still have senders,
continuing to drain...

For example, in one of my tests, this line was printed 31602 times
within 1.28 seconds.

<img width="1097" height="344" alt="image"
src="https://github.com/user-attachments/assets/b3aff30c-596f-4837-a081-fbe165ba6254"
/>

This:
1. slightly complicates debugging for our engineers
2. adds costs and confusion for customers who turn on
`DD_LOG_LEVEL=debug` to debug the extension



## This PR
Add a timer and print this line at most once every 100ms, so it will be
printed at most 20 times within the 2-second spindown time.

## Testing
No testing for now. Should be straightforward. Will see if logs are
reduced in future debugging.

Author: lym953

bottlecap/src/logs/agent.rs

@@ -1,4 +1,5 @@
 use std::sync::Arc;
+use std::time::{Duration, Instant};
 use tokio::sync::mpsc::{self, Sender};
 use tokio_util::sync::CancellationToken;
 use tracing::debug;
@@ -9,6 +10,8 @@ use crate::logs::{aggregator_service::AggregatorHandle, processor::LogsProcessor
 use crate::tags;
 use crate::{LAMBDA_RUNTIME_SLUG, config};
 
+const DRAIN_LOG_INTERVAL: Duration = Duration::from_millis(100);
+
 #[allow(clippy::module_name_repetitions)]
 pub struct LogsAgent {
     rx: mpsc::Receiver<TelemetryEvent>,
@@ -57,6 +60,7 @@ impl LogsAgent {
                     debug!("LOGS_AGENT | Received shutdown signal, draining remaining events");
 
                     // Drain remaining events
+                    let mut last_drain_log_time = Instant::now().checked_sub(DRAIN_LOG_INTERVAL).expect("Failed to subtract interval from now");
                     'drain_logs_loop: loop {
                         match self.rx.try_recv() {
                             Ok(event) => {
@@ -68,7 +72,12 @@ impl LogsAgent {
                             },
                             // Empty signals there are still outstanding senders
                             Err(tokio::sync::mpsc::error::TryRecvError::Empty) => {
-                                debug!("LOGS_AGENT | No more events to process but still have senders, continuing to drain...");
+                                // Log at most once every 100ms to avoid spamming the logs
+                                let now = Instant::now();
+                                if now.duration_since(last_drain_log_time) >= DRAIN_LOG_INTERVAL {
+                                    debug!("LOGS_AGENT | No more events to process but still have senders, continuing to drain...");
+                                    last_drain_log_time = now;
+                                }
                             },
                         }
                     }