STARTUP TUNING: Warn on slow clock source

## 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