fix: elasticsearch indexer streaming and flushing

See merge request https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer/-/merge_requests/668



Merged-by: Dmitry Gruzd <dgruzd@gitlab.com>
Approved-by: Dmitry Gruzd <dgruzd@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: Arturo Herrero <arturo.herrero@gmail.com>