Aggs: add a metric to track accepted/rejected scripted_metric aggs if the allow lost is enabled

https://github.com/elastic/elasticsearch/pull/109444 added an allow list that permits only a selected set of scripts to be run as part of a scripted_metric aggregation.

It'd be useful to know how many times users were blocked from running a scripted_metric agg when the allow list is enabled, and how many times they weren't.