[DPE-7603][DPE-6990] Fix check in relation initialisation

[marceloneppel]

Issue

The handler for the database_requested event creates a new user through the primary, but checks if the current unit is has started PostgreSQL. The current unit may be the leader but not the primary, which may lead to errors when trying to access the database.

There is also the case where, for some reason, the K8s PG primary service stops redirecting traffic to the unit for a short period. This makes the charm understand that the cluster has initialised completely and can create users (but it may not due to that weird situation). This still needs further investigation.

Solution

Use the right condition (primary endpoint being ready) in the check.

Fixes #1011
Fixes #927

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.33%. Comparing base (fa52899) to head (39eb2b9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1054   +/-   ##
=======================================
  Coverage   73.33%   73.33%           
=======================================
  Files          15       15           
  Lines        3919     3919           
  Branches      573      573           
=======================================
  Hits         2874     2874           
  Misses        831      831           
  Partials      214      214           

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[dragomirp]

We'll most probably need to remove the old cla-check to be able to merge.

[taurus-forever]

Wow, this is a nice catch!