Property to disable percentile calculations

[benjchristensen]

If someone does not want to use percentile calculations there is no point in tracking and calculating them so let's have a property to disable.

[pgrinchenko]

This would be a good one. +1

[benjchristensen]

The metrics.rollingPercentile.enabled property controls this: https://github.com/Netflix/Hystrix/wiki/Configuration

It already existed but a modification is coming shortly to make it more efficiently prevent any work done inside RollingPercentile when metrics poll it.