Improve Scaling documentation for Streaming

[ThisIsMissEm]

Changes to: https://github.com/mastodon/documentation/blob/master/content/en/admin/scaling.md#streaming-api-streaming

• Should include a note that for Prometheus based monitoring, you need to monitor each individual streaming server instance, not the nginx endpoint, as the request to nginx would be routed to an arbitrary streaming server instance, which would mess with your metrics and not give you the complete picture.

(@andypiper feel free to assign this to me if you want)

[andypiper]

Thanks - happy to review when a PR is available. Appreciated!