Is the `gateway-metadata` endpoint optional in `jupyter-controller` charm?

`latest/edge` version of the charm that was recently published to Charmhub shows it requires the [`gateway-metadata` endpoint](https://charmhub.io/jupyter-controller/integrations?channel=latest/edge#gateway-metadata), but there are no known providers for it.

- Is this endpoint's metadata missing `optional: true`?
- If it is not optional, what can be integrated at this endpoint?

https://github.com/canonical/notebook-operators/blame/06adc5ef98cfc5dd050d745a33caeb3abb790ce9/charms/jupyter-controller/metadata.yaml#L32

Example run in Test Observer: https://test-observer.canonical.com/#/charms/408346?testExecutionId=559480&testResultId=11568011

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Filed on behalf of @canonical/solutions-qa

Start with charms/jupyter-controller/metadata.yaml at the linked endpoint definition, then compare its metadata with the Charmhub integrations page and the Test Observer result. Determine whether gateway-metadata should be optional or identify the supported provider and integration; done means the endpoint's expected behavior is confirmed and the metadata or explanation is updated accordingly.

Written by the indexing model from the issue text.