We are adding a query parameter to the field_caps api in order to filter out
fields with no values. The parameter is called `include_empty_fields` and
defaults to true, and if set to false it will filter out from the field_caps
response all the fields that has no value in the index.
We keep track of FieldInfos during refresh in order to know which field has
value in an index. We added also a system property
`es.field_caps_empty_fields_filter` in order to disable this feature if needed.
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Co-authored-by: Matthias Wilhelm <ankertal@gmail.com>
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