chore: Add a timer to avoid repeated debug logs

[lym953]

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.

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.