metadata.yaml doesn't set a limit on database relations

[amc94]

Bug Description

it's possible to relate multiple databases to the databases endpoint, leading to hook failed: storage-attached

To Reproduce

deploy with mysql-router and mysql and relate both to the database endpoint

Environment

https://test-observer.canonical.com/#/charms/275993?testExecutionId=287586&testResultId=7944781

Relevant log output

2025-11-18T16:04:19.264Z [container-agent] 2025-11-18 16:04:19 DEBUG jujuc server.go:302 running hook tool "juju-log" for target/0-uploads-storage-attached-7ce6e585ee5de350da0010ccd0fe6552
2025-11-18T16:04:19.264Z [container-agent] 2025-11-18 16:04:19 ERROR juju-log Uncaught exception while in charm code:
2025-11-18T16:04:19.264Z [container-agent] Traceback (most recent call last):
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/./src/charm.py", line 1580, in <module>
2025-11-18T16:04:19.264Z [container-agent]     main(WordpressCharm)
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/main.py", line 39, in main
2025-11-18T16:04:19.264Z [container-agent]     return _main.main(charm_class=charm_class, use_juju_for_storage=use_juju_for_storage)
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/_main.py", line 504, in main
2025-11-18T16:04:19.264Z [container-agent]     manager.run()
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/_main.py", line 488, in run
2025-11-18T16:04:19.264Z [container-agent]     self._emit()
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/_main.py", line 423, in _emit
2025-11-18T16:04:19.264Z [container-agent]     self._emit_charm_event(self.dispatcher.event_name)
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/_main.py", line 467, in _emit_charm_event
2025-11-18T16:04:19.264Z [container-agent]     event_to_emit.emit(*args, **kwargs)
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/framework.py", line 351, in emit
2025-11-18T16:04:19.264Z [container-agent]     framework._emit(event)
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/framework.py", line 924, in _emit
2025-11-18T16:04:19.264Z [container-agent]     self._reemit(event_path)
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/framework.py", line 1036, in _reemit
2025-11-18T16:04:19.264Z [container-agent]     custom_handler(event)
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/./src/charm.py", line 1565, in _reconciliation
2025-11-18T16:04:19.264Z [container-agent]     self._core_reconciliation()
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/./src/charm.py", line 924, in _core_reconciliation
2025-11-18T16:04:19.264Z [container-agent]     if not self._current_effective_db_info:
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/./src/charm.py", line 675, in _current_effective_db_info
2025-11-18T16:04:19.264Z [container-agent]     relation = self.model.get_relation(self._DATABASE_RELATION_NAME)
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/model.py", line 277, in get_relation
2025-11-18T16:04:19.264Z [container-agent]     return self.relations._get_unique(relation_name, relation_id)
2025-11-18T16:04:19.264Z [container-agent]   File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/model.py", line 1025, in _get_unique
2025-11-18T16:04:19.264Z [container-agent]     raise TooManyRelatedAppsError(relation_name, num_related, 1)
2025-11-18T16:04:19.264Z [container-agent] ops.model.TooManyRelatedAppsError: Too many remote applications on database (2 > 1)

Additional context

Part of the charm qa testing effort

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The internal ticket has been created: https://warthogs.atlassian.net/browse/ISD-4784.

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