Add jitter and connections accepted/initiated counters to TCP sampler

[WUMUXIAN]

Problem
Via BPF, add jitter (median deviation of srtt) and two counters for the TCP sampler

• connection accepted
• connection initiated

Solution
Added the above mentioned metrics by introducing new kernel probes in BPF.

Result
Three new metrics introduced to the TCP sampler.

• tcp/jitter - histogram
• tcp/connection/accepted - counter
• tcp/connection/initiated - counter

[brayniac]

Overall, this looks good. I'd like to see updates to the metrics documentation docs/METRICS.md to go with the new metric. Also see the comment about the name of the new enum variant.

We can ignore the failing 'audit' check, I will address separately as it's unrelated to this PR - also unrelated is that the runs on 'ubuntu xenial' seem to be stuck queued forever. I think it's safe to ignore both these CI issues.

Please address the docs and my comment on the enum variant and then I'll be happy to merge