Added gated serde support for metadata behind serde feature flag

[dustins]

I had a use case for needing to (de)serialize the metadata tags. It was a relatively simple code change to add support since there aren't any exotic types being used and any overhead can be hidden behind a feature flag.

I'm relatively new and unfamiliar with much of the project so if there are other obvious parts of core that could benefit please let me know and I could add them.

[abique]

If you add serde to the tags, it implies that the library also guarantee the tags stability, so a tag that you serialize today, you'll be able to de-serialize it in 10 years.

It isn't a light decision.

[dustins]

Thanks for the input! I appreciate your concern but I'm not sure I agree that providing serialization capabilities infers any additional stability. As a library consumer I wouldn't expect any stronger guarantees than what the library API would provide based on versioning scheme. But maybe that is a faulty assumption on my part 🙂

[abique]

At least it should be clarified in the documentation.

[dustins]

I'll make that change to reflect the expectations of stability. Ultimately it'll be up to the project to decide which of our assumptions they align more closely with but being more explicit about stability guarantees is an excellent suggestion!