Want a metadata-only pool dump feature

[rincebrain]

Describe the feature would like to see added to OpenZFS

As remarked in e.g. #16520, it'd be useful to have an equivalent of xfs_metadump to emit only the non-L0 data blocks of a pool, so one could somewhat efficiently capture things for debugging without requiring you keep the whole pool around.

How will this feature improve OpenZFS?

Making it more practical to investigate metadata-based flaws without requiring people keep TB of data laying around.

Additional context

I think you could get a reasonable approximation with something like convincing dump_blockstats in zdb to shim zio_read to zio_write all the BPs to a shadow pool copy that has sparse files as the members, but then you probably need to remember to forge the labels and also convince various codepaths not to freak out about missing data blocks on later examination, which probably means something like an import option and a spa_t setting...