[PGBouncer] Adds support for 1.8 new metrics

[nmuesch]

What does this PR do?

Pulls in PGBouncer column names when running query to more easily identify available metrics. Adds in new metrics to be submitted.

Motivation

Users ran into an issue when using PGBouncer 1.8. There are now more metrics available for "SHOW STATS" and "SHOW POOLS"

Previous metrics available - http://web.archive.org/web/20170623115443/https://pgbouncer.github.io/usage.html
Newly available metrics - https://pgbouncer.github.io/usage.html

Since we were indexing the columns using a specific order, the addition of new metrics caused the check to crash.

Testing Guidelines

An overview on testing
is available in our contribution guidelines.

Versioning

• Bumped the check version in manifest.json
• Bumped the check version in datadog_checks/{integration}/__init__.py
• Updated CHANGELOG.md. Please use Unreleased as the date in the title
  for the new section.

Additional Notes

Instead of just submitting every column available as a metric, I continue to use the SHOW STATS and SHOW POOLS object so that we have a finer control over what metric names and types are when they are submitted. Should future versions provide additional metrics, or change names, we just need to add to those two objects.

Finally, some metric names changed between pre 1.8 and post. Due to this, there may be duplicate metric names in the SHOW STATS object, but with different key/column names.

[nmuesch]

Closing in favor of #1016