:param metric: Specification of the mass matrix, either as a
vector consisting of the diagonal elements of the covariance
matrix ('diag' or 'diag_e') or the full covariance matrix
('dense' or 'dense_e').
If the value of the metric argument is a string other than
'diag', 'diag_e', 'dense', or 'dense_e', it must be
a valid filepath to a JSON or Rdump file which contains an entry
'inv_metric' whose value is either the diagonal vector or
the full covariance matrix.
If the value of the metric argument is a list of paths, its
length must match the number of chains and all paths must be
Also the fit object has metric and stepsizemethods.
Then you just need to unpack metric items to individual files and have a list of files.