Add postgres custom metrics

[ipolishchuk]

This pull request is a replacement for #69
It fixes a typo in the londiste3 last_seen metric definition and adds another example of a custom metrics tx_duration_92. There are two metrics here postgresql.max_tx_duration - largest transacrion duration, and postgresql.max_query_duration -- largest query duration for active sessions. The tx_duration_92 metrics are for postgres v9.2+
An extra value of tx_duration_92, it provides a working format sample for a metric without descriptor.

[talwai]

Upstream dd-agent PR: DataDog/dd-agent#1395

[LeoCavaille]

We probably want to merge this, but re-work it a bit to allow more liberty in defining the custom metrics dictionary rather than enabling pre-written metrics like londiste3_last_seen or tx_duration_92. People should be able to define that in the class and fill the template not the other way around IMO.
Putting it in standby for now and it would probably helped by a refactoring of the classes.
Feel free to use it on top of the current release if needed.

[sethcleveland]

Custom metric support provided here -- #224.

[truthbk]

Superceded by #224 - closing. Thanks regardless @ipolishchuk 🙇