Documentation of metadata types

[brancomat]

The official documentation (https://arpa-simc.github.io/arkimet/) lacks a description of all metadata types.

There's also some 10-years-old doc made by @enrenr and @spanezz here: https://github.com/ARPA-SIMC/arkimet/tree/master/doc/scan that goes a bit further describing how the single metadata type is composed for different formats that seems useful and could be incorporated in the official documentation.

And that brings us to the age-old issue of how to be sure that the doc is updated each time the code is updated (spoiler alert: the doc isn't updated).

[spanezz]

I'm attempting to do it by implementing arki-dump --doc-metadata, with the intention to keep each metadata item's documentation as close as possible to its implementation

[spanezz]

We have an initial inventory and documentation attempt at https://github.com/ARPA-SIMC/arkimet/blob/master/doc/metadata.rst

This is updated manually by running something like ./run-local arki-dump --doc-metadata > doc/metadata.rst. On the implementation side, I added a write_documentation method to all metadata item implementations in arki/types. The presence of the documentation method is enforced by the compiler. I tried to store documentation, where possible, alongside the class definitions in header files.

Some metadata elements are more documented than others: for example, I don't really know what the ODIM specific metadata refers to, and I'm not the right person to add stable references to the WMO Manual on Codes to annotate GRIB and BUFR values.

This might be something that grows over time, now that we have a place to write this documentation, and possibly with the help of domain-specific experts.