[Stan 2.34] expose sampler argument "save_metric"

[mitzimorris]

Summary:

Add functionality to save metric as JSON to CmdStanPy.

Description:

CmdStan PR stan-dev/cmdstan#1203 added new adaptive sampler argument save_metric. Make this available from CmdStanPy:

• Expose adaptive sampler argument save_metric
• Add methods to CmdStanMCMC to access the saved metric and stepsize.
• Document use and use cases.

Current Version:

This will be available in 3.34

[WardBrian]

Should we expose it, or just always enable it by default?

The files are never that large relative to the output csvs

[mitzimorris]

always enable by default. what are we doing with save_config? we should be consistent.

[WardBrian]

I think save_cmdstan_config will always be enabled if it is supported. Eventually I think it makes sense to move essentially all the config logic to reading that file and then ignore the comments in the output csvs entirely.