track_total_hits for multi-search API

[kavilla]

Is your feature request related to a problem? Please describe.
https://opensearch.org/docs/latest/opensearch/rest-api/multi-search/ accepts track_total_hits in the body but that means all my searches have to include track_total_hits within the body. Unlike https://opensearch.org/docs/latest/opensearch/rest-api/search/, which accepts track_total_hits as a query string.

When we toggle “Batch concurrent searches” within OpenSearch Dashboards, the application uses _msearch throughout it however then it no longer shows total hits. It just defaults to the limit which, out of the box, is 10,000.

Describe the solution you'd like
_msearch accepts track_total_hits as a query string and then applies to all the search queries.

Describe alternatives you've considered
Default track_total_hits to be true for searches. But not sure about the performance of this. From my understanding this was how the legacy engine was prior to 7.x

Additional context
This comes from: https://discuss.opendistrocommunity.dev/t/opensearch-dashboards-shows-10000-as-hits-total/8397/30

In OpenSearch Dashboards, we could make sure when _msearch is used when we add the track_total_hits to the body but I don't have the insight to see why we couldn't accept track_total_hits and do a broad apply within _msearch. If there is a blocker then we can close this.

[matthewdeanmartin]

I am also having a hard time getting "track_total_hits" to have any effect. I set the window to 50,000 up from 10,000 results- I can page thru 50,000 results, but hit count is always 10,000 or less. I'll update again if I can work out a smallest reproducible example.

[mhoydis13]

I am also having a hard time getting "track_total_hits" to have any effect. I set the window to 50,000 up from 10,000 results- I can page thru 50,000 results, but hit count is always 10,000 or less. I'll update again if I can work out a smallest reproducible example.

Hey @matthewdeanmartin , try toggling off “Batch concurrent searches”. I found, as detailed in the forum thread linked below, that feature seems to uniquely affect remote cross-search connected clusters.
https://forum.opensearch.org/t/opensearch-dashboards-shows-10000-as-hits-total/8397/31?u=mhoydis