Add getter for data for `Metadata` and `FlatVar`

[ph-kev]

In ClimaCalibrate, there is code that are written like this:

https://github.com/CliMA/ClimaCalibrate.jl/blob/31cda9c43db6352772229ebf8e059ed1ff67916a/ext/ClimaAnalysisExt.jl#L61C13-L67C23

Add a function like get_data would improve the formatting of the code.