[Postgresql] logging bug for custom metrics

[orenmazor]

This is actually a combination of two bugs:

1. If you use custom metrics and one of your queries has too many results, you will discover a typo where we attempt to log to 'self.warning', which is not a thing.
2. the brackets on that call to self.warning result in attempting to format the function result rather than the parameter.

[LeoCavaille]

Hey @orenmazor thanks a lot for contributing and catching that.
For 1., the self.warning method actually exists, it's meant to display the warning on the info page and in Datadog.

For 2., you're completely right this is a typo. Also it looks like the CI is failing because you have a flake8 error. You can run rake lint (see also how to setup a dev env) before pushing.

[orenmazor]

Fixed! I didnt have flake8 built into vim in this env so I didn't catch that.

makes sense about self.warning. this was a flyby fix on one of my servers that I just caught. not super familiar with dd-agent.

[LeoCavaille]

@orenmazor thanks, merged in a rebased version as 071d06f in #1634