Fix "I'm feeling lucky" query and add metrics

[Kixiron]

This snippet states that the query was made for a vastly different docs.rs, one that had 280 crates with docs. We now have over 200k crates, so this should probably get fixed. Additionally, metrics for it would be interesting, because why not?

[jyn514]

That part of the code is only run if the query is empty, i.e. someone hit "I'm feeling lucky" without putting anything in the search box. In that case I think selecting a random crate is fine.

[Kixiron]

It's definitely far from perfect, as it can panic if the offset is off. For example, my test db only has ~50 crates right now, so since the offset is calculated with FLOOR(RANDOM() * 280) that always panics except when RANDOM() is 0

[Kixiron]

The math of my last comment may be a wee off, but point stands, it may even be possible (if horrifically improbable) that it panics in prod

[syphar]

This was fixed in #1251 and #1260, so it could be closed.