[8.x] Add source mode stats to MappingStats (#117463)

[martijnvg]

Backports the following commits to 8.x:

• Add source mode stats to MappingStats (Add source mode stats to MappingStats #117463)

[github-actions]

Documentation preview:

• ✨ Changed pages

[martijnvg]

The 8.15.6 bwc ci job failed with the following transient failure:

* What went wrong:
Could not determine the dependencies of task ':qa:full-cluster-restart:v8.15.6#bwcTest'.
> Could not resolve all dependencies for configuration ':qa:full-cluster-restart:es_distro_extracted_bwc_8.15.6'.
   > Could not find elasticsearch-distribution:elasticsearch:8.15.6.
     Searched in the following locations:
       - https://artifacts-no-kpi.elastic.co/downloads/elasticsearch/elasticsearch-8.15.6-linux-x86_64.tar.gz
     Required by:
         project :qa:full-cluster-restart

The other 37 bwc ci jobs completed successfully Ignoring this. Ignoring this failure.