Rate limiting affects server administrator

*This issue was originally created by [**@boogewooge**](https://github.com/boogewooge) at .*

### Background information

- **Dendrite version or git SHA**: 0.8.9
- **Monolith or Polylith?**: Monolith
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: No
- **`go version`**: 1.18.4
- **Client used (if applicable)**: Element Web/Desktop

In https://github.com/matrix-org/dendrite/issues/2589, where `rate_limiting` being enabled causes issues, a server administrator (more than one in fact) was also affected, though https://github.com/matrix-org/dendrite/blob/main/dendrite-sample.monolith.yaml#L182= indicates that shouldn't be the case.

Start with the rate_limiting configuration in dendrite-sample.monolith.yaml around line 182 and trace how administrator requests are identified in the monolith. Reproduce the reported behavior using Dendrite 0.8.9 with Postgres, then verify that enabling rate limiting leaves server administrator requests unaffected.

Written by the indexing model from the issue text.