[Monitoring] Once the buffer has any events, the collector is always ready - #36995

Fixes #36991

Copied from the issue:

The work done in #36153 introduced an issue with one collectors isReady implementation - the stats collector within monitoring. The event roller is designed to accept events, then flush them and it's possible it doesn't have any at any given time. This is causing an issue with the /api/stats?extended endpoint where we are waiting for all collectors to be ready before returning usage data and the experience, because of this bug, is if you repeatably ping /api/stats?extended it will occasionally return a 503 indicating stats are not ready yet.

To test, repeatably hit the /api/stats?extended endpoint and ensure that once it returns a 200, it always return a 200.

My test bash script:

for ((i=1;i<=100;i++));
do
  curl -I --show-error --fail --header "Content-type: application/json" -- "elastic:changeme@localhost:5601/api/stats?extended";
  sleep 1s
done

Note: I'm not sure we can add a test for this, as this configuration circumvents the potential 500 response