Disable default metric collection

[dominics]

There are a bunch of default metrics that the agent tries to collect that aren't relevant for collecting postgres metrics from Aptible. Many of these can apparently be turned off by removing the .default files in /etc/dd-agent/conf.d

For example:

root@deadbeef:/etc/dd-agent/conf.d# ls -l *.default
-rw-r--r-- 1 root root  525 Feb 13 19:30 agent_metrics.yaml.default
-rw-r--r-- 1 root root 2118 Feb 13 19:30 disk.yaml.default
-rw-r--r-- 1 root root  790 Feb 13 19:30 network.yaml.default
-rw-r--r-- 1 root root  329 Feb 13 19:30 ntp.yaml.default

Not a major concern, but just annoying to see irrelevant failures in the logs like:

IOError: [Errno 2] No such file or directory: '/host/proc/filesystems'

[dominics]

Ideally, the "core" metrics from e.g. unix.py would be disabled too, but this might require a bit more surgery.