movePrimary needs to clear the filtering metadata on the new primary shard in case the CSS holds a leftover entry marking the collection as UNSHARDED when it is in fact SHARDED with no chunks on the new primary shard.

To avoid leaving corrupted in-memory state behind, we decided that movePrimary would unconditionally clear the filtering metadata on the new primary for all tracked collections.

However, this has implications for migrations, which do not expect their filtering metadata to be cleared while they are in progress and may consequently fail with a non-retryable error.

To avoid aborting migrations and to reduce the impact of these clears, movePrimary will now clear the filtering metadata only when it is known and marked UNSHARDED. In all other cases the metadata is already up to date and is not owned/driven by movePrimary, so it is left untouched.