Re: Add per-backend lock statistics

Hi Bertrand,

I reviewed and tested v2; it builds cleanly. The implementation closely
mirrors the existing per-backend IO/WAL stats (same flush path, struct
layout, and backend-type guard), and the hot path is untouched: the
per-backend counters piggyback on the existing global counting points
(pgstat_count_lock_waits / pgstat_count_lock_fastpath_exceeded), so they
only fire where pg_stat_lock already counts -- fastpath_exceeded when the
fast-path slot limit is exceeded, and waits/wait_time only after a wait
longer than deadlock_timeout.

One tiny nit: in pgstat_count_backend_lock_waits() and
pgstat_count_backend_lock_fastpath_exceeded(), the Assert() is directly
followed by the counter update. A blank line after the Assert() would read
a bit better and is the more usual style in this code.

Otherwise LGTM.

Regards,
Rui