APM Server under Elastic Agent does not shutdown gracefully sometimes

[endorama]

Please include configurations and logs if available.

For confirmed bugs, please report:

• Version: At least 8.15.4 (likely earlier)-> 9.0.0-beta
• Operating System: Linux
• Discuss Forum URL:
• Steps to Reproduce: none

I've not been able to consistently reproduce this issue, but has been observed multiple times through logs on Elastic Cloud Hosted.

Sometimes APM Server does not shutdown correctly following the graceful shutdown procedure. The observed behavior is a shutdown being initialized and no additional logs, which suggests an hard termination of the process.

APM Server has been inspected for possible bugs in our graceful shutdown logic but none was found. From my local test runs, in all cases APM Server (standalone) and managed was able to correctly handle a SIGTERM on Elastic Agent process, or the removal of the APM policy from the Elastic Agent policy.

This issue share some similarities with #6875 but is happening on a different OS (evaluation on Windows has not been performed).