MetricBands
How to read a metric's value - the boundary between "fine", "worth a look", and "act on this".
Parameters
fair
Value at which the metric stops being unremarkable.
poor
Value at which it warrants action.
higherIsWorse
True when a larger number is a worse result. Governs which side of the thresholds counts as trouble, so a caller never has to hardcode the direction per metric.