User-Space, Statically Defined Tracing (USDT)

[HiHat]

This PR adds documentation for User-Space, Statically Defined Tracing (USDT) as well as tracepoints (including documentation and usage examples).

User-space, Statically Defined Tracing (USDT) allows for more observability during development, debugging, code review, and production usage. The tracepoints make it possible to keep track of custom statistics and enable detailed monitoring of otherwise hidden internals and have little to no performance impact when unused. Linux kernels (4.x or newer) can hook into the tracepoints and execute eBPF programs in a kernel VM once the tracepoint is called.

This PR includes, for example, tracepoints for in- and outbound P2P messages.

The two main eBPF front-ends with support for USDT are bpftrace an BPF Compiler Collection (BCC). BCC is used for complex tools and daemons and bpftrace is preferred for one-liners and shorter scripts. Example tracing scripts for both are provided with this PR.

This PR adds following tracepoints:

• net:inbound_message
• net:outbound_message
• net:inbound_connection
• net:outbound_connection
• net:closed_connnection
• net:evicted_inbound_connection
• net:misbehaving_connection
• valildation:block_connected
• mempool:added
• mempool:removed
• mempool:replaced
• mempool:rejected

See doc/tracing.md and contrib/tracing/ for documentation and example tracing scripts.