More coverage in COS CI upgrade tests

[MichaelThamm]

We currently have little coverage in our COS integration tests. We could significantly improve coverage with 2 additional tests:

1. Coordinater charms nginx directive assertions
   • We can access the nginx config file in the coordinator with jssh --container nginx mimir/0 "cat /etc/nginx/nginx.conf"
   • e.g. Mimir's config
   • then curl each of these endpoints and ensure they route correctly, e.g. request.code == 200
2. otelcol pipeline assertion
   • feat: Check otelcol logs in COS tests #170
   • TL;DR, ensure there are no ERROR and (in some cases) WARNING logs in the internal otelcol logs
   • i.e. no operational issues in COS since Otelcol is the central aggregator