Update database_peak_analysis.md to document CPU time by queryid
Update database_peak_analysis.md to document CPU time by queryid. We should document pg_stat_kcache_exec_total_time
as a viable alternative to pg_stat_statements_seconds_total
See example in https://gitlab.com/gitlab-org/gitlab/-/issues/499633#note_2162270460.
Edited by Thong Kuah