Proper instrumentation to better understand performance of the attack

[0xffiber]

As of now the functionality of TargetStats is quite limited.

Ideally we want to know much more. For example:

• histogram for connection lifetime (better with per-stage markers, like TCP SYN, TLS handshake, SOCKS5 CONNECT, etc)
• histogram for number of packets sent within a single connection lifetime (to understand if increasing/decreasing RPC makes a difference)
• performance per proxy (when in use) not only per target
• min/max/avg/histograms for all measurements
• more

From architecture perspective, there's no need to implement own instrumentation stack. Existing libraries could be utilize, e.g. Prometheus client library without external emitters.