Fetching _seq_no via `fields` results in 5xx error about elasticsearch HOT 3 OPEN

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Notice that the same issue does not happen if we request _seq_no via stored_fields because StoredFieldPhase does not use ValueFetcher to retrieve field values.

Does it mean that _seq_no gets returned? I think the field is not stored, hence it gets ignored? I am asking because this could inform the fix to apply: do we actually want to throw an error, or should we treat it like a situation where a non existing field is requested?

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@salvatore-campagna - do you know the answer to @javanna 's question above? We have an action to work on this but need to determine what fix to apply.

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