This change adds a grok_pattern field to the GET categories API
output in ML. It's calculated using the regex and examples in the
categorization result, and applying a list of candidate Grok
patterns to the bits in between the tokens that are considered to
define the category.
This can currently be considered a prototype, as the Grok patterns
it produces are not optimal. However, enough people have said it
would be useful for it to be worthwhile exposing it as experimental
functionality for interested parties to try out.
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