Optional endpoints are not marked as optional

The Charm QA program run by @canonical/solutions-qa attempts to test every endpoint of every public charm on Charmhub. We rely on charm metadata to determine what is required to deploy a charm.

Edit: I'm leaving my original description below, but I think this issue should ultimately reflect that the metadata should be updated to reflect optionality for all optional endpoints.

Original description:
The `mongodb` charm offers/`provides` the `cluster` endpoint and `config-server` interface, but this endpoint is not marked as optional, and there are no publicly listed charms that `require` this endpoint, so our tests can't do anything.

1. Is the `cluster` endpoint required, or could it be marked optional in the metadata?
```
provides:
cluster:
interface: config-server
optional: true
```
2. Is there a private charm somewhere that does `require` the `cluster` endpoint, but that private charm should actually be made public?

Inspect the mongodb charm metadata for the cluster/config-server endpoint and compare it with the endpoints used by the Charm QA program. Confirm which endpoints are optional and whether any consumers are missing; done when metadata marks all optional endpoints appropriately and QA can distinguish required endpoints.

Written by the indexing model from the issue text.