Integrate a database monitoring dashboard

Description

GitLab includes a Sidekiq dashboard in the admin panel from which you can view the health of your sidekiq cluster, queues, etc. Something similar would be useful for our postgresql, mysql and redis servers too.

Proposal

For postgresql, introduce https://github.com/ankane/pghero

For redis, https://github.com/BaseSecrete/redis_dashboard ?

For mysql, I'm not sure what the options are.

I self-host a mastodon node which comes with PgHero built in, and it's really handy from an administrator's point of view. Probably not too useful for GitLab.com - but who knows?

Links / references

Documentation blurb

Overview

What is it? Why should someone use this feature? What is the underlying (business) problem? How do you use this feature?

Use cases

Who is this for? Provide one or more use cases.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

/cc @DouweM