Issue in availability metrics in 5k GET GitLab Omnibus installation
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### Summary
The dashboards of a fresh installed 5k environment does not show information correctly.
Already created in GET: gitlab-environment-toolkit#467
### Steps to reproduce
Install a 5k environment via the GET.
### What is the current *bug* behavior?
```
avg_over_time(pg_up{job="postgres",instance=~"$instance"}[$__interval]) or avg_over_time(up{job="postgres",instance=~"$instance"}[$__interval])
```


### What is the expected *correct* behavior?
```
avg(
avg_over_time(pg_up{job="postgres",instance=~"$instance"}[$__interval]) or avg_over_time(up{job="postgres",instance=~"$instance"}[$__interval])
)
```


### Relevant logs
-
### Details of package version
Will update soon to latest 15.x version. But in 14.9 this is present, maybe also in the newest version.
```
||/ Name Version Architecture Description
+++-==============-============-============-============================================================
un gitlab-ce <none> <none> (no description available)
ii gitlab-ee 14.9.4-ee.0 amd64 GitLab Enterprise Edition (including NGINX, Postgres, Redis)
```
### Environment details
* Operating System: `Ubuntu 20.04`
* Installation Target, remove incorrect values:
* VM: Openstack
* Installation Type, remove incorrect values:
* New Installation
* Multinode (5k reference)
### Configuration details
-
issue