We can leverage the composite agg's new `missing_bucket` feature on
terms groupings. This means the aggregation criteria used in the indexer
will now return null buckets for missing keys.
Because all buckets are now returned (even if a key is null),
we can guarantee correct doc counts with
"combined" jobs (where a job rolls up multiple schemas). This was
previously impossible since composite would ignore documents that
didn't have _all_ the keys, meaning non-overlapping schemas would
cause composite to return no buckets.
Note: date_histo does not use `missing_bucket`, since a timestamp is
always required.
The docs have been adjusted to recommend a single, combined job. It
also makes reference to the previous issue to help users that are upgrading
(rather than just deleting the sections).
- All rollup pages should be marked as experimental instead of just
the top page
- While the job config docs state which aggregations are allowed, adding
a section which specifically details this in one place is more convenient
for the user
- Add a clarification that the DeleteJob API does not delete the rollup
data, just the rollup job.