Need doc for PPL count by span function [DOC]

[smacrakis]

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Olly generated a query including the stats span function (stats count() by span(memory, 100000)) -- which seems to work --, but I can't find any documentation on count/span in the stats section or anywhere else in the PPL documentation.

*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.

2.15

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