The input and output files should both be vdb files. The grids are the list of grids that should have mipmaps generated. Standard usage would probably be to provide a vdb file and give all of the grids so that they are all mipmapped, but per-grid decisions can also be made. Grids that are not included in the list will be written to the output file as they are in the input file.

The metadata argument allows for metadata from the original grids to be copied to the coarse mipmap grids that are generated. All of the strictly necessary metadata is generated automatically, so there isn't a specific requirement here unless GridGroups are being used. Beyond that, I assumed it might be useful to copy metadata in case there are tools that use grid metadata down the line.

For GridGroups, the work is a bit involved. First, the original grids will have to have a label corresponding to the name of the group (e.g. cluster_0) and an integer `index` that is unique to the grid. Both of those metadata keys also need to be copied to the coarse grids using the meta argument (-meta label -meta index).