Include wait_event in Marginalia sampling

This came out of https://gitlab.com/gitlab-org/gitlab/-/issues/334638#note_619764455.

# curl -s http://localhost:9187/metrics  | grep pg_stat_ac | grep marg
# HELP pg_stat_activity_marginalia_sampler_active_count Number of active queries at time of sample
# TYPE pg_stat_activity_marginalia_sampler_active_count gauge
pg_stat_activity_marginalia_sampler_active_count{application="",command="SELECT",endpoint="",server="/var/opt/gitlab/postgresql:5432",state="active",usename="gitlab-psql",wait_event="",wait_event_type=""} 1
pg_stat_activity_marginalia_sampler_active_count{application="",command="SELECT",endpoint="",server="/var/opt/gitlab/postgresql:5432",state="idle",usename="gitlab",wait_event="ClientRead",wait_event_type="Client"} 2
pg_stat_activity_marginalia_sampler_active_count{application="sidekiq",command="SELECT",endpoint="Geo::VerificationBatchWorker",server="/var/opt/gitlab/postgresql:5432",state="idle",usename="gitlab",wait_event="ClientRead",wait_event_type="Client"} 13
pg_stat_activity_marginalia_sampler_active_count{application="web",command="SELECT",endpoint="",server="/var/opt/gitlab/postgresql:5432",state="idle",usename="gitlab",wait_event="ClientRead",wait_event_type="Client"} 1
pg_stat_activity_marginalia_sampler_active_count{application="web",command="SELECT",endpoint="POST /api/:version/jobs/request",server="/var/opt/gitlab/postgresql:5432",state="idle",usename="gitlab",wait_event="ClientRead",wait_event_type="Client"} 1
# HELP pg_stat_activity_marginalia_sampler_max_tx_age_in_seconds Number of active queries at time of sample
# TYPE pg_stat_activity_marginalia_sampler_max_tx_age_in_seconds gauge
pg_stat_activity_marginalia_sampler_max_tx_age_in_seconds{application="",command="SELECT",endpoint="",server="/var/opt/gitlab/postgresql:5432",state="active",usename="gitlab-psql",wait_event="",wait_event_type=""} 0.001609
pg_stat_activity_marginalia_sampler_max_tx_age_in_seconds{application="",command="SELECT",endpoint="",server="/var/opt/gitlab/postgresql:5432",state="idle",usename="gitlab",wait_event="ClientRead",wait_event_type="Client"} NaN
pg_stat_activity_marginalia_sampler_max_tx_age_in_seconds{application="sidekiq",command="SELECT",endpoint="Geo::VerificationBatchWorker",server="/var/opt/gitlab/postgresql:5432",state="idle",usename="gitlab",wait_event="ClientRead",wait_event_type="Client"} NaN
pg_stat_activity_marginalia_sampler_max_tx_age_in_seconds{application="web",command="SELECT",endpoint="",server="/var/opt/gitlab/postgresql:5432",state="idle",usename="gitlab",wait_event="ClientRead",wait_event_type="Client"} NaN
pg_stat_activity_marginalia_sampler_max_tx_age_in_seconds{application="web",command="SELECT",endpoint="POST /api/:version/jobs/request",server="/var/opt/gitlab/postgresql:5432",state="idle",usename="gitlab",wait_event="ClientRead",wait_event_type="Client"} NaN