Add limit_type to M_RESOURCE_LIMIT_EXCEEDED error code

Author: hughns

proposals/0000-resource-limit-exceeded-extra-fields.md

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+# MSC0000: Add limit_type to M_RESOURCE_LIMIT_EXCEEDED error code
+
+## Proposal
+
+1. Add an optional `limit_type` field to the body of the [`M_RESOURCE_LIMIT_EXCEEDED`] error response that has the same
+definition as it does for [`m.server_notice`]:
+
+> The kind of usage limit the server has exceeded
+
+And on the possible values:
+
+> The specified values for `limit_type` are:
+> `monthly_active_user` The server’s number of active users in the last 30 days has exceeded the maximum. New
+> connections are being refused by the server. What defines “active” is left as an implementation detail, however
+> servers are encouraged to treat syncing users as “active”.
+
+2. Update the description of `admin_contact` so that instead of "a place to reach out to" that it is the same definition
+as the `admin_contact` field on the [`m.server_notice`] which says "a URI giving a contact method for the server
+administrator".
+
+This amounts to unifying the representation of a server resource limit being exceeded in an error code and the server
+notice.
+
+## Potential issues
+
+None anticipated. Optional extra fields on the error.
+
+## Alternatives
+
+None considered.
+
+## Security considerations
+
+None.
+
+## Unstable prefix
+
+Ideally implementations would use an unstable prefix of something like `io.element.mscXXX.limit_type` whilst the
+MSC is in development.
+
+However, it is noted that the implementation in Synapse has existed since
+[2018](https://github.com/matrix-org/synapse/pull/3707) and so the unprefixed `limit_type` is being used in the wild.
+As such some clients also support this unprefixed version.
+
+## Dependencies
+
+None.
+
+[`M_RESOURCE_LIMIT_EXCEEDED`]: https://spec.matrix.org/v1.16/client-server-api/#other-error-codes
+[`m.server_notice`]: https://spec.matrix.org/v1.16/client-server-api/#mroommessagemserver_notice