[postgres] Allow 'host' config value be UNIX socket path

[DylanFrese]

The PostgreSQL check has been changed to try to connect to the PostgreSQL database via a UNIX socket if the hostname provided in the configuration begins with a forward-slash ('/').

This behaviour is similar to the 'psql' command-line utility. From psql(1):

  --host=hostname
      Specifies the host name of the machine on which the server is
      running. If the value begins with a slash, it is used as the
      directory for the Unix-domain socket.

With the difference being that we interpret it as an exact path, whereas psql interprets it as a directory to look for a socket with a particularly formatted name.

This addresses issue #2361. Arguably, a new configuration parameter, unix_sock or similar, should be added. I chose to do it this way as it touches less code, and is, as mentioned above, similar to how psql does things.

The documentation at Datadog/documentation/content/integrations/postgresql.md should also be updated with the new content of postgres.yaml.example.

[truthbk]

Thanks for this @DylanFrese code looks good, we'll talk over whether introducing a new unix_sock parameter makes sense, or if if this is - as it seems - more than enough. Thanks a bunch!

[antifuchs]

I just ran into the problem that this fixes - any chance this could be pulled in sometime? (:

[olivielpeau]

Thanks @DylanFrese, just gave your changes a test run and everything works nicely! Keeping host makes sense to me too, thanks for the thorough explanation.

Merging, this will be part of the next minor release, 5.10.0

Travis is failing but I think the current master fixes the failures. I'll keep an eye on the CI once this is merged