Disk I/O profiling with eBPF

Coroot's node-agent now includes an eBPF disk I/O profiler, on by default, that attributes block-request time and bytes read or written to application stacks. It hooks submit_bio and bio_endio rather than scheduler off-CPU events, and samples at most one request per millisecond per CPU, weighted by the requests skipped. The author's tests on one 12-core machine (pgbench, fio) show up to 1.6% lower throughput, and the figures are self-reported. A retention-job case on a Percona cluster traced heavy reads to cold-page deletes.