Support multiple measures in `MemorySampler`

It would be handy to be able to record multiple measures at once in `MemorySampler`. Particularly, recording `process` and `managed_spilled` at the same time gives you a picture of both RAM and disk usage. But also tracking any of the metrics could be nice.

This is easy to do. The one small question is how to structure the DataFrame, which currently has a timeseries index, and one column per `sample` call. This effectively adds another dimension (`measure`) to the data, so using an xarray Dataset would actually be nicest here, though we could also just make a DataFrame in tidy format [like xarray does](https://docs.xarray.dev/en/stable/user-guide/pandas.html#hierarchical-and-tidy-data).

To be used in https://github.com/coiled/coiled-runtime/issues/191.