specification: user-defined statistics

[wkalt]

Related to https://github.com/foxglove/mcap/issues/384

It would be nice if there were a way for writers to define and use custom statistics in their mcap files and have them surfaced by the "info" subcommand. I think this could be implemented with a new record type like "custom statistic", which could minimally contain "name" and "value" fields. Better IMO would be to provide a little more structure, like supporting "channel statistic" and "file statistic" variants, or even "channel statistic", "file statistic", "attachment statistic", "metadata statistic", "chunk statistic". The channel statistics would have channel IDs associated with them, file statistics would be whole-file, and metadata/attachment/chunk statistics could contain references to the relevant record offset. I am imagining all these records would be written to the summary section somewhere, I guess in a new statistics section. The "info" command could then display file and channel statistics, and the list attachments/chunks/metadata/channels commands could show the relevant type of statistic as well.

Some additional things to think about:

• what about within-chunk channel statistics?