Support the ability to filter metrics by instrument_action

[gmckeever]

For metrics automatically reported, offer the ability to ignore metrics by a certain action. Currently you can create additional metrics by action, but not ignore them if they are already reported by default.

ex. `exclude_action :create'

[nextmat]

What is the use case here? Are we trying to remove a specific action from the aggregate metrics or doing something like instrument_action :all and then exclude a few?

[gmckeever]

@nextmat pointed out offline that there is already an issue for this #48 which covers the same use case (filtering an action from the aggregate metrics). I'll close this issue out.