[postgres] Made BGW_METRICS version dependent

[ipolishchuk]

We are using Postgres 9.1.9 and I see a lot of errors in collector,log like this:

2014-12-22 18:33:47 UTC | WARNING | dd.collector | checks.postgres(postgres.py:277) | Not all metrics may be available: ('ERROR', '42703', 'column "checkpoint_write_time" does not exist')

The problem is that postgres 9.2 columns of a pg_stat_bgwriter view are hardcoded in postgres.py
The columns checkpoint_write_time, checkpoint_sync_time, buffers_backend_fsync are missing in version 9.1
After a BGW_METRICS query failed, all the following queries fail, too.

The proposed fix makes the BGW_METRICS Postgres version aware. This may not be the most elegant solution, but I tried to make as fewer changes as possible, and do everything in the spirit of the existing code.

After I applied this fix, I have BGW_METRICS working, and the queries following BGW_METRICS are working too.

[LeoCavaille]

@ipolishchuk thanks a lot for your contribution. We will review it shortly and consider its inclusion in the next agent release.

[LeoCavaille]

@ipolishchuk this is really great, I tested your change with postgres 8.4. 9.0, 9.1, 9.2, 9.3, 9.4 (we are in the process of improving integration testing and running tests against a larger matrix of versions but we are not there yet...). And it works great. One thing that I realized though is buffers_backend_fsync is only available after 9.1.
Merging this change and I will add something to handle the pre-9.1 versions.
👍

cc @alq666