Refactor pg_stat_get_lock() to use a helper function

Refactor pg_stat_get_lock() to use a helper function

This commit extracts the tuple-building logic from pg_stat_get_lock()
into a new static helper pg_stat_lock_build_tuples().  This is in
preparation for a follow-up patch, to add support for backend-level lock
stats, which will reuse the same helper.

This change follows the pattern established by pg_stat_io_build_tuples()
for IO stats and pg_stat_wal_build_tuple() for WAL stats.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Tatsuya Kawata <kawatatatsuya0913@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Tristan Partin <tristan@partin.io>
Reviewed-by: Rui Zhao <zhaorui126@gmail.com>
Discussion: https://postgr.es/m/aiAzEY+cMQb/W8yu@bdtpg