CI configuration requires it

this is absolutely the wrong way to do it

This doesn't fix anything yet but maybe running non-ancient versions will make
it easier to fix something.

Update servant-prometheus library to get conforming metric names

Add note about getting the PrivateStorageio environment first

Got it to compile!
This could be chinese for all I know.

The author of the library used says the tests currently fail.
This might be a problem with how Nix handles things.

Also, client-side quantile computation is not available
anymore.  We'll just user server-computed histograms instead.