feat(s2n-quic-core): add USDT probe support

[camshaft]

Description of changes:

This change adds support for defining static probes throughout the codebase:

s2n-quic/quic/s2n-quic-core/src/buffer/receive_buffer/probe.rs

Lines 4 to 18 in cd06c58

	crate::probe::define!(
	extern "probe" {
	/// Emitted when a buffer is allocated for a particular offset
	#[link_name = s2n_quic_core__buffer__receive_buffer__alloc]
	pub fn alloc(offset: u64, capacity: usize);
	/// Emitted when a chunk is read from the beginning of the buffer
	#[link_name = s2n_quic_core__buffer__receive_buffer__pop]
	pub fn pop(offset: u64, len: usize);
	/// Emitted when a chunk of data is written at an offset
	#[link_name = s2n_quic_core__buffer__receive_buffer__write]
	pub fn write(offset: u64, len: usize);
	}
	);

In the code, you can then just call your probes like regular functions - all with autocompletion and strict typing!

s2n-quic/quic/s2n-quic-core/src/buffer/receive_buffer/request.rs

Lines 64 to 65 in cd06c58

	pub fn write(self, buffer: &mut BytesMut) {
	super::probe::write(self.offset, self.data.len());

The probe can then be configured through feature flags to:

• emit a tracing record
• register as a USDT probe, which can be read by tools like bpftrace.

Testing:

I've enabled probe-tracing on the s2n-quic dev-dependencies, so all of the integration tests can be more-easily traced:

$ S2N_LOG=trace cargo test -p s2n-quic client_server -- --nocapture | rg receive_buffer::probe
    Finished test [unoptimized + debuginfo] target(s) in 0.11s
     Running unittests src/lib.rs (target/debug/deps/s2n_quic-f075e86750846335)
0:00:00.050000 s2n_quic_core::buffer::receive_buffer::probe::alloc: offset=0 capacity=4096
0:00:00.050000 s2n_quic_core::buffer::receive_buffer::probe::write: offset=0 len=234
0:00:00.050000 s2n_quic_core::buffer::receive_buffer::probe::pop: offset=0 len=4
0:00:00.050000 s2n_quic_core::buffer::receive_buffer::probe::pop: offset=4 len=230
0:00:00.100000 s2n_quic_core::buffer::receive_buffer::probe::alloc: offset=0 capacity=4096
0:00:00.100000 s2n_quic_core::buffer::receive_buffer::probe::write: offset=0 len=90
0:00:00.100000 s2n_quic_core::buffer::receive_buffer::probe::pop: offset=0 len=4
0:00:00.100000 s2n_quic_core::buffer::receive_buffer::probe::pop: offset=4 len=86
0:00:00.100000 s2n_quic_core::buffer::receive_buffer::probe::alloc: offset=0 capacity=4096
0:00:00.100000 s2n_quic_core::buffer::receive_buffer::probe::write: offset=0 len=600
0:00:00.100000 s2n_quic_core::buffer::receive_buffer::probe::pop: offset=0 len=4
0:00:00.100000 s2n_quic_core::buffer::receive_buffer::probe::pop: offset=4 len=88
...

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.