monitoring: add DataMayBeNaN to ratio alerts, fix wording

Created by: bobheadxi

Add DataMayBeNaN to ratio alerts, since the denominator of these queries (typically some form of "total requests") can be 0. For example, index_queue_growth_rate has been firing frequently due to sum(increase(src_index_queue_processor_total[30m])) == 0. This issue happens most frequently with *_queue_growth_rate alerts it seems

frequency comparison for index_queue_growth_rate:

Also removes some redundant "for ..." from metric descriptions, since this is appended by the generator already.

closes https://github.com/sourcegraph/sourcegraph/issues/12868

follows up https://github.com/sourcegraph/sourcegraph/pull/12756