Adds postgres IDs sequence capacity saturation monitoring

Prabakaran Murugesanrequested to merge
add_pg_id_sequences_saturation_monitoring into master

pg_sequences metrics was introduced in gitlab-exporter v15.1.0 in gitlab-org/ruby/gems/gitlab-exporter!212 (merged).

It exposes gitlab_pg_sequences_min_value, gitlab_pg_sequences_max_value and gitlab_pg_sequences_current_value metrics for each sequence.

This MR adds a saturation monitoring for all the ID sequences, with soft SLO: 50% and hard SLO: 90%.

Note: It's very similar to pg_int4_id.libsonnet, except it covers both int4 and int8 ID sequences.

Issue: gitlab-org/gitlab#439940 (closed)

Edited by Prabakaran Murugesan

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