STARTUP TUNING: Warn on slow clock source

[jackshirazi]

Is your feature request related to a problem?

A slow clock source (like xen) impacts how long nanoTime() call takes, can add a second to startup!

Describe the solution you'd like

Add check for clock source and warn if it's a slow one. Note, can be after startup since the information is for subsequent runs anyway

Describe alternatives you've considered

Only alternative is to remove all the timing stats during startup. Actually, that's a feasible tuning option to add - I'll leave it to a later discussion when this gets prioritized

Additional context

https://github.com/elastic/apm-agent-java/issues/2349

[tobiasstadler]

@jackshirazi Wagt do you think about #2471

[jackshirazi]

@tobiasstadler that's an excellent idea!