monitoring: allow NaN values in alert queries by default

Created by: bobheadxi

Many alert queries divide-by-0 in normal situations, such as low-traffic instances and scenarios, causing many alerts to require DataMayBeNaN to be true. This change switches our default to allow NaN values without triggering alerts.

There are currently 44 instances of alerts with DataMayBeNan: true many of them ratio alerts, and possibly more require it: https://github.com/sourcegraph/sourcegraph/issues/14401 was due to histogram_quantile returning NaN on no data, and it seems that all histogram_quantile alerts are susceptible to this (1, 2) and require DataMayBeNaN (there are 23 alerts using histogram_quantile), so at this point might as well switch the defaults around

Closes https://github.com/sourcegraph/sourcegraph/issues/10689