Separate normal and deprioritized crates in the queue size metric

[pietroalbini]

Now that we implemented automatic deprioritization for large projects (#714) we should extend the metrics exposed by docs.rs to our monitoring system to account for that.

I'd love for the docsrs_queued_crates_count metric to be split into two:

• docsrs_queued_crates_count with the queued crates that have a neutral or positive priority
• docsrs_deprioritized_queued_crates_count with the queued crates that have a negative priority

This would allow us to define alerting rules such as "alert when there are more than 50 non-prioritized crates in the queue, or more than 300 deprioritized crates".

[Kixiron]

I believe I fixed this in #774 I didn't, I was thinking of a different thing