I don't think you can round values from a pure Elasticsearch query, neither from the templating language (Handlebars) used by Kibana to format text strings in actions like email.
In any case, I also don't think you can use aggregations in the Elasticsearch Query alert, according to this statement from this other post by @Patrick_Mueller
The elasticsearch query rule is the other "generic" rule type, but does not yet support aggregations, so I don't think you can use that one either.
Check the rest of the comment for some further advice.