Add support to gather indices stats for unloaded segments

[danielmitterdorfer]

This is related to elastic/elasticsearch#39512 - we should add an option to the IndexStats telemetry device to also gather information about unloaded segments so we can measure how much memory a frozen index would use if were unfrozen (this will only work from Elasticsearch 7.1.0 onwards though).

Marking as "blocked" as this feature is not yet available in Elasticsearch.

[danielmitterdorfer]

The feature is now merged and available on the master and 7.x branch in Elasticsearch.

[danielmitterdorfer]

Frozen indices are deprecated as of Elasticsearch 7.14 thus closing this issue.