--------

- Add the ability to configure custom FCM options, which is necessary for using iOS with Firebase. ([\#145](https://github.com/matrix-org/sygnal/issues/145))
- Add a Prometheus metric (`sygnal_inflight_request_limit_drop`) that shows the number of notifications dropped due to exceeding the in-flight concurrent request limit. ([\#146](https://github.com/matrix-org/sygnal/issues/146))