The other crates in this repository use zenithdb/rust-postgres as a
dependency for the related items, instead of the crates.io versions.
Switching to using that for the proxy as well removes an additional
three dependencies when we compile. (319 -> 316)
This patch has been extracted from #348, where it became unnecessary
after we had decided that we didn't want to measure anything inside
PostgresBackend.
IMO the change is good enough to make its way into the codebase,
even though it brings nothing "new" to the code.
The metrics are served by an http endpoint, which
is meant to be spawned in a new thread.
In the future the endpoint will provide more APIs,
but for the time being, we won't bother with proper routing.
