[Heartbeat] Fix concurrency errors with multiple ports

[andrewvc]

What does this PR do?

Fixes #16917 , where in heartbeat 6.x multiple TCP ports cause a crash with configs like this one.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/uptime (:uptime)

[urso]

I presume this is a workaround for 6.8 only? Instead of one DNS lookup, this change will trigger a DNS lookup per port number, if actual host names are configured.

[andrewvc]

@urso that's correct about the DNS lookup. I'm OK with that, as I doubt that's a limiting factor in most cases, and I don't think a more extensive restructuring of the code is warranted in 6.x

[andrewvc]

Hmmm, there is some additional problem here that is causing additional test failures due to checks not running. Investigating.