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[BUGFIX] Storage: errors from a single secondary querier should be warnings. This is a backport of #14496 to release-2.54 branch. #13434 introduced an unwanted change in behaviour: if there was no primary querier and a single secondary querier, the secondary would be treated like a primary. This PR restores the previous behaviour, that all secondary queriers report errors as warnings. In order to test this behaviour, I changed `TestMergeQuerierWithSecondaries_ErrorHandling` so it now calls `NewMergeQuerier` rather than creating the internal data structure directly. This in turn required all the data types to change, so I merged `mockGenericQuerier` into `mockQuerier`. Also replaced `unwrapMockGenericQuerier` with a visitor pattern. While I was there, I addressed the comment from https://github.com/prometheus/prometheus/pull/13434#pullrequestreview-2191058921 to short-circuit the merge of single querier with any number of no-op or nil queriers. Signed-off-by: Bryan Boreham <bjboreham@gmail.com>