Kibana should have a strict mode that fails on Elasticsearch deprecation warnings

[LeeDr]

Describe the feature: Kibana should run tests in a mode where Kibana or the tests fail if we get deprecation warnings from Elasticsearch on requests. Since we couldn't immediately address every deprecation warning we would also need a way to skip or ignore a list of warnings until they were addressed.

Describe a specific use case for the feature: There was a recent issue on Cloud, and customers running through a proxy where the connection was closed after hitting some number of warnings.

Here's one example of the type of warnings; #22484

And the user problems https://discuss.elastic.co/t/bad-gateway-errors-in-discover-in-kibana-6-4-on-some-indices/146576

Elasticsearch issues: elastic/elasticsearch#33479
elastic/elasticsearch#33508

[jbudz]

I looked into this, I'd propose a new configuration elasticsearch.logDeprecations (log level = warn) that is true in dev. These are available in response headers from elaticsearch, and currently only available in the callback style response in the elasticsearch-js client ref. The next version will support this in promise style, so I'm going to hold off for now so we can avoid multiple low level changes.