Compare brick and function measurements to threshold rules.

Configuration has two maps from metric key to a vector of rules:
:brick-thresholds for brick metrics and :function-thresholds for each
function's metrics. Each rule is a map with a :rule type, a :value, and an
optional :level (:error, the default, or :warning):

- {:rule :max :value n} flags a value above n.
- {:rule :min :value n} flags a value below n.
- {:rule :std-devs :value k} flags a brick whose metric is more than k
  standard deviations above the mean of the other bricks of the same type
  (components are compared with components, bases with bases). It needs
  at least :min-peers other bricks (default 3) and is skipped otherwise.
  Brick thresholds only.

Brick rules take an optional :types, a set of brick types (:component,
:base) the rule applies to. Rules skip metrics with no value.