reliability: Determine and document SLOs for the agent

[kakkoyun]

cc @metalmatze

[brancz]

In my mind, the most important thing is that Parca Agent can unwind stacks successfully and attempts to send profiling data (note that I intentionally don't say that it succeeds because that is outside of its responsibility).

Being able to measure this is most important, setting goals is good, but understanding it is most important I think and we can then use the current state to set goals to improve.