Add Maxabs, Minabs to statistic in monitors

[alexhutko]

Add Maxabs, Minabs to "statistic" in monitors.

Basically I want to say:
Alert me if the abs(value) of metric ever exceeds X for any channel for any single measurement (i.e. asap).

I think that is currently only possible with 2 monitors:

1. alert me if the maximum of metric is greater than +THRESH over the last 5 hours.
2. alert me if the minimum of metric is less than -THRESH over the last 5 hours.

Let me know if I'm dumb and wrong and there's a smart way to do this.