The maximum number of fields per index is limited to 1000 by default by the
`index.mapping.total_fields.limit` setting to prevent accidental mapping
explosions due to too many fields. Currently all metadata fields also count
towards this limit, which can lead to some confusion when using lower limits.
It is not obvious for users that they cannot actually add as many fields as
are specified by the limit in this case.
This change takes the number of metadata fields out of the field count that we
check against the field limit. It also adds tests that check that we can add
fields up to the specified limit, but throw an exception for any additional field added.
Closes#24096
eafc2a5470