[DPE-3292] test mongos relation (#341)

## Issue
Validation of mongos/cluster relation doesn't exist

## Solution
Test that relation does what its supposed to do i.e. user creation /
removal

## Files to not review
Feel free to ignore the test charm we copied over to
`tests/integration/sharding_tests/application/`, this should be removed
in the futre

## Future work
Ideally we can deploy a "Test Charm" for mongos to be hosted on instead
of this current charm that we have copied over
`tests/integration/sharding_tests/application/`. We can either:
1. test with `data-integrator` when it works with `mongos`
2. publish a [test-charm](https://charmhub.io/mysql-test-app) and deploy
it in the tests

.github/workflows/ci.yaml

@@ -83,6 +83,8 @@ jobs:
           - metric-integration
           - sharding-integration
           - sharding-relation-integration
+          - sharding-race-conditions-integration
+          - sharding-mongos-integration
     name: ${{ matrix.tox-environments }}
     needs:
       - lint

lib/charms/mongodb/v0/mongodb.py

@@ -1,4 +1,5 @@
 """Code for interactions with MongoDB."""
+
 # Copyright 2023 Canonical Ltd.
 # See LICENSE file for licensing details.
 

lib/charms/mongodb/v0/mongodb_secrets.py

@@ -1,4 +1,5 @@
 """Secrets related helper classes/functions."""
+
 # Copyright 2023 Canonical Ltd.
 # See LICENSE file for licensing details.
 

lib/charms/mongodb/v1/helpers.py

@@ -1,4 +1,5 @@
 """Simple functions, which can be used in both K8s and VM charms."""
+
 # Copyright 2023 Canonical Ltd.
 # See LICENSE file for licensing details.
 import json

lib/charms/mongodb/v1/mongos.py

@@ -1,4 +1,5 @@
 """Code for interactions with MongoDB."""
+
 # Copyright 2023 Canonical Ltd.
 # See LICENSE file for licensing details.
 

lib/charms/mongodb/v1/users.py

@@ -1,4 +1,5 @@
 """Users configuration for MongoDB."""
+
 # Copyright 2023 Canonical Ltd.
 # See LICENSE file for licensing details.
 from typing import Set

src/config.py

@@ -1,4 +1,5 @@
 """Configuration for MongoDB Charm."""
+
 # Copyright 2023 Canonical Ltd.
 # See LICENSE file for licensing details.
 

src/machine_helpers.py

@@ -1,4 +1,5 @@
 """Machine Charm specific functions for operating MongoDB."""
+
 # Copyright 2023 Canonical Ltd.
 # See LICENSE file for licensing details.
 import logging

tests/integration/sharding_tests/application/charmcraft.yaml

@@ -0,0 +1,11 @@
+# Copyright 2023 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+type: charm
+bases:
+  - build-on:
+      - name: "ubuntu"
+        channel: "22.04"
+    run-on:
+      - name: "ubuntu"
+        channel: "22.04"