Colin Goodheart-Smithe 2c12c3e628 Add _bucket_count option to buckets_path
This change adds a new special path to the buckets_path syntax
`_bucket_count`. This new option will return the number of buckets for a
multi-bucket aggregation, which can then be used in pipeline
aggregations.

Closes #19553
2016-07-26 09:28:21 +01:00
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