Saving extra metadata in signature format

[mr-eyes]

In snipe, I wanted to keep the number of sketched bases to assess the sketching efficiency. However, there is no place in the sourmash signature JSON to hold this information so I had to add a custom suffix to the signature name. It would be great if we can add a metadata dict to the sourmash signature.

[bluegenes]

@luizirber suggested a generic metadata field, but notes "The danger of generic metadata with key/val is that you can NEVER depend on the value actually being there, so any code using those values need to account for that"

@ccbaumler additionally wants info on whether a sketch is a pangenome sketch
@bluegenes wants to add information on whether or not a sketch is a translated sketch

[ctb]

concern: metadata mayget out of date. could tie metadata to md5hash of original data file or something?

plugins for manipulating metadata would be great!

related issues:

• Potential modifications to signature format for major releases #268
• optionally (by default?) store the actual k-mers in the signatures #211 (could store k-mers in metadata 😭 )
• Provide a standard format / provenance info for commands used in data preprocessing #269
• add number of bp & sequences => signature file #246

I think one place we talked about mechanisms for this that were unrelated to modifying Signature was here: #2180

[mr-eyes]

I would like to also mention #2985 here.

[ccbaumler]

Pangenome-related metadata could also be the count of genomes that have been compressed into the pangenome. This is an important metric to define the reliability of the pangenome element characterization.

[bluegenes]

#2219

[bluegenes]

@luizirber suggests that the metadata field could be open to users to modify, not used internally in sourmash. If we want to store and use a field internally, we can create actual individual fields (so each is guaranteed to have an entry w/specific meaning).