Add gauge for total fields mapped in an index

[tragiclifestories]

Motivation: ES imposes a limit on the number of fields mapped, and we'd like to alert when we're approaching that limit. We've got a lot of custom ELK tooling and suchlike, so there's the potential for a bug to flood the index with new fields, for example.

Happy to bring a PR if it is thought to be useful!

[zwopir]

Hi James,

thanks for the feature proposal. I'm not sure if this a feature the majority of our users would benefit from. In general we are trying to focus on the ES operational metrics (as compared to the content related metrics). The above use case has clearly a bit of both.

What would be the underlying API call to get the number of fields mapped in an index? maybe we are already doing the call and are just not exposing the result as a metric?

[tragiclifestories]

I don't think there is an underlying API to get this number directly - you have to get the mapping data from the index API and count the fields. I might be wrong about that though.

(EDIT: and there will be plenty of edge cases there, eg when you have a text field with a subfield you'll have to count that twice ... )

[svenmueller]

The metric would be very helpful for us too (e.g. we haven't noticed the increase in mapped fields) which now results in a very poor Kibana UI performance

[msodi]

May someone review #364 ?