Add filter_path option to Elasticsearch query alert rules

[A-Hall]

Describe the feature:

Currently there is no way to manipulate the response body in an alert action in Kibana alerts, for example to slim down {{context.hits}} to only fields relevant to the alert. That leaves us with formatting the actual hits array sent back to Kibana from ES as the only option. You can reduce the fields included in the _source by including "_source": ["myRelevantField"] in the alert query, but that still returns the metadata fields. The only way to remove the metadata fields and other fields is with filter_path as a URL parameter, which currently isn't' available as an option with queries generated with Kibana alerts. Something like this would allow you to strip down the response body so it's not more verbose than it needs to be.

Edited: Assumed that using "_source": ["myRelevantField"] or "fields": [ "node_stats.process.cpu.percent"] would reduce the fields returned in the hits array, but apparently Kibana strips those from the query (maybe due to some verification that they're not passing?).

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)