Drop the deprecated /metrics endpoint

[jtpio]

Originally posted by @manics in #73 (comment)

If you're renaming the package this might be a good opportunity to make any breaking changes? e.g. following up from #68 /metrics could be completely removed instead of just deprecated?

We should consider doing this for the next major release, so adding to the 0.5.0 milestone.

[yuvipanda]

YESSS. LET US KILL IT.