Instrument new idle_time metric introduced on ecto_sql 3.3.0

[GPrimola]

ecto_sql v3.3.0 introduced a new metric idle_time that measure the time the connection spent waiting before being checked out for the query

[ackerdev]

@GPrimola I've implemented this along with a few other changes in a fork here: https://github.com/theblitzapp/prometheus-ecto/tree/idle-and-repo-labels

Check the PR linked above to see what else is changed; there are some breaking changes. The idle_time changes are in a single commit, so if you want to avoid the breaking changes it should be simple to set up your own fork and cherry-pick my commit.