Disallow disabling _field_names

[jpountz]

Now that _field_names has basically no overhead anymore (#26930), we should ignore the enabled setting in 6.x (and log deprecation) and reject it in 7.x, or maybe 8,x if we want to maintain compatibility for templates.

cc @colings86

[colings86]

Caveat to this: _fieldNames has basically no overhead in the case where doc_values and norms are enabled. If doc_values are disabled then _field_names will have the same overhead as before.
I wonder if for a high index through put use case you would still want the option to disable _field_names because you are disabling doc_values on fields that you don't want to aggregate to increase indexing throughput and don't want to counter this by adding the overhead of writing to _field_names?

[javanna]

@elastic/es-search-aggs

[ppf2]

As part of this ticket, can we also clarify the documentation to mention that there is much less overhead now for most use cases except for the caveats mentioned above? Thx!

[jimczi]

We discussed this issue internally and agreed that this is something that we still want to do.
Since it's not high priority we also said that we should at least clarify the documentation to mention the new cost of the _field_names field so I am adding the >docs and adoptme label.

[cbuescher]

I just checked the docs that @ppf2 linked to above and the section about disabling _field_names has been removed from the "Tune for indexing speed" page as of #37717.
I will check if there are other locations in the docs where it would make sense to mention the cost of _field_names. That should cover the docs side of things for now, we would still need to deprecate the enabled setting separately.

[cbuescher]

I think what @colings86 rewrote in #31029 covers the documentation needs here, so I'm going to remove that lable, leaving the deprecation issue open.