Proposal

EncOOOXOR was added as part of OOO support to indicate that this chunk, while XOR encoded, belongs to the OOO part of the Head block. This is used only in the ChunkDiskMapper when m-mapping so that when Prometheus restarts we can identify which chunks belong to the OOO Head.

Since this is not a new encoding, we should simplify this and remove EncOOOXOR, and instead do the same bit in ChunkDiskMapper itself to identify OOO chunks. This will also simplify things when we have OOO support for native histograms.

This won't be a breaking change because the same bit will be set to 1 in the ChunkDiskMapper; it is just how we do it is changing.

This issue is broken down from #11329