monitoring: Allow alerting on zero

Created by: ryanslade

We changed LessOrEqual and GreaterOrEqual to pointers so that we can determine the difference between a missing value and a value of zero.

We also introduced a "fluent" interface to make it easer to define alerts now that they contain pointers.

For example,

Alert{GreaterOrEqual: 10, For: 10*time.Second}

becomes

Alert().GreaterOrEqual(10).For(10*time.Second)