This change adds the `after_key` of a composite aggregation directly in the response.
It is redundant when all buckets are not filtered/removed by a pipeline aggregation since in this case the `after_key` is always the last bucket
in the response. Though when using a pipeline aggregation to filter composite buckets, the `after_key` can be lost if the last bucket is filtered.
This commit fixes this situation by always returning the `after_key` in a dedicated section.

* es/master:
  [Docs] Fix explanation for `from` and `size` example (#28320)
  Adapt bwc version after backport #28358
  Always return the after_key in composite aggregation response (#28358)
  Adds test name to MockPageCacheRecycler exception (#28359)
  Adds a note in the `terms` aggregation docs regarding pagination (#28360)
  [Test] Fix DiscoveryNodesTests.testDeltas() (#28361)
  Update packaging tests to work with meta plugins (#28336)
  Remove Painless Type from MethodWriter in favor of Java Class. (#28346)
  [Doc] Fixs typo in reverse-nested-aggregation.asciidoc (#28348)
  Reindex: Shore up rethrottle test
  Only assert single commit iff index created on 6.2
  isHeldByCurrentThread should return primitive bool
  [Docs] Clarify `html` encoder in highlighting.asciidoc (#27766)
  Fix GeoDistance query example (#28355)
  Settings: Introduce settings updater for a list of settings (#28338)
  Adapt bwc version after backport #28310

* es/6.x:
  [Docs] Fix explanation for `from` and `size` example (#28320)
  Adapt bwc version after backport #28358
  Always return the after_key in composite aggregation response (#28358)
  Adds a note in the `terms` aggregation docs regarding pagination (#28360)
  Update packaging tests to work with meta plugins (#28336)
  Remove Painless Type from MethodWriter in favor of Java Class. (#28346)
  [Doc] Fixs typo in reverse-nested-aggregation.asciidoc (#28348)
  [Docs] Fixed Indices information breaking changes (#27914)
  Reindex: Shore up rethrottle test
  isHeldByCurrentThread should return primitive bool
  [Docs] Clarify `html` encoder in highlighting.asciidoc (#27766)
  Only assert single commit iff index created on 6.2
  Deprecate the `update_all_types` option. (#28284)
  Fix GeoDistance query example
  Settings: Introduce settings updater for a list of settings (#28338)
  [Docs] Fix asciidoc style in composite agg docs
  Adapt bwc version after backport #28310
  Adds the ability to specify a format on composite date_histogram source (#28310)

* master: (23 commits)
  Update Netty to 4.1.16.Final (elastic#28345)
  Fix peer recovery flushing loop (elastic#28350)
  REST high-level client: add support for exists alias (elastic#28332)
  REST high-level client: move to POST when calling API to retrieve which support request body (elastic#28342)
  Add Indices Aliases API to the high level REST client (elastic#27876)
  Java Api clean up: remove deprecated `isShardsAcked` (elastic#28311)
  [Docs] Fix explanation for `from` and `size` example (elastic#28320)
  Adapt bwc version after backport elastic#28358
  Always return the after_key in composite aggregation response (elastic#28358)
  Adds test name to MockPageCacheRecycler exception (elastic#28359)
  Adds a note in the `terms` aggregation docs regarding pagination (elastic#28360)
  [Test] Fix DiscoveryNodesTests.testDeltas() (elastic#28361)
  Update packaging tests to work with meta plugins (elastic#28336)
  Remove Painless Type from MethodWriter in favor of Java Class. (elastic#28346)
  [Doc] Fixs typo in reverse-nested-aggregation.asciidoc (elastic#28348)
  Reindex: Shore up rethrottle test
  Only assert single commit iff index created on 6.2
  isHeldByCurrentThread should return primitive bool
  [Docs] Clarify `html` encoder in highlighting.asciidoc (elastic#27766)
  Fix GeoDistance query example (elastic#28355)
  ...