Adds packet drops counter to TCP sampler

[rittme]

Problem

Add a probe to measure TCP packet drops.

Solution

Added a BPF kernel probe to collect the TCP packet drops metrics.

Result

A new metric introduced to TCP sampler: tcp/drop

[CLAassistant]

All committers have signed the CLA.

[WUMUXIAN]

Looks good to me.

[brayniac]

Thanks for the PR!

What's the benefit of using BPF for this metric? We already have TX and RX drops by reading from basic telemetry sources (https://github.com/twitter/rezolus/blob/master/docs/METRICS.md#basic-4) Trying to understand what this offers that isn't already covered.

[WUMUXIAN]

Thanks for the PR!

What's the benefit of using BPF for this metric? We already have TX and RX drops by reading from basic telemetry sources (https://github.com/twitter/rezolus/blob/master/docs/METRICS.md#basic-4) Trying to understand what this offers that isn't already covered.

My understanding of this TX and RX drops are at network device level, it's not necessarily TCP drops. This counter counts packets dropped in the TCP stack, it can be an indicator of two many duplicates, out of order packets, short lived sessions and etc.

[rittme]

@brayniac, thanks for your review! I tried making the docs clearer. Let me know if this works for you.

[brayniac]

Looks good to me.