Disable platform metrics by default/make platform metrics opt-in, rather than opt-out

[asachs01]

Expected Behavior

As a Sensu operator, I should be able to operate the application without worrying about the disk on my system filling up.

Current Behavior

Currently, the default behavior for platform metrics is set to false, meaning that platform metrics are logged...without a log-rotation rule. The result is that users have this enabled and often don't realize it, leading to disk space being consumed and impacting the system's stability

Possible Solution

Platform metrics should be opt-in, rather than opt-out