Elasticsearch Indexer failed with error code 413 (Request Entity Too Large)

One customer reported that they started seeing errors on their sidekiq workers after upgrading to 12.8, bulk request 2236: error: elastic: Error 413 (Request Entity Too Large)

@nick.thomas suggested lowering the values of two related configurations, which fixed their issue.

The default configuration for these two parameters are https://gitlab.com/gitlab-org/gitlab/-/blob/v12.8.7-ee/ee/app/models/ee/application_setting.rb#L101-102. The default bulk concurrency is 10. The bulk size parameter default value in RoR is 10.megabytes and it's actually equal to 10* 2^10 * 2^10 (while is refereed as mebibyte at other places). On the other hand, the Elasticsearch http.max_content_length defaults to 100mb. So, there could be mismatch between Megabyte vs Mebibyte.

This MR provides more information about how these two parameters are plumbed into the underlying indexer. gitlab-elasticsearch-indexer!56 (merged)

CC: @wchandler @jrreid @phikai