* PMM-11984: Add new metrics `pg_available_extensions` and `pg_extensions` to expose info about available extensions and installed extensions accordingly
* PMM-11984: Exclude comment from the metric
* PMM-11984: Fix small issues
* PMM-11984: Update label values
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Co-authored-by: Alexey Mukas <91831719+ritbl@users.noreply.github.com>