rfcs: fine-grained cpu attribution

CRDB lacks primitives to attribute CPU usage to specific scopes (ranges,
tenants, sessions, statements) or processes (pebble compactions,
snapshot generation). For subsystems that rely on such attribution, we
use proxy signals like '# of batch requests' or 'size of read/write
request' as observed by a range/replica or a given tenant. We propose
using a patched Go runtime to track CPU use at the level of individual
goroutines, using the primitive to derive an accurate resource
consumption signal in various CRDB subsystems. The compiler, language,
and tooling will remain the same.

Release note: None