Why are we renaming the exported type from MetadataKey to ReactiveMetadataKey?

I plan to get rid of DataKey as a distinct thing from MetadataKey and instead make it StaticMetadataKey while the thing we used to call just MetadataKey is now ReactiveMetadataKey, but I'm saving that for a followup PR

Could you please elaborate on how this change makes things more ergonomic in the commit description. It's not immediately clear to me how and there's no test/examples demonstrating the change.

Its more ergonomic because you don't have to pass two functions every time you create a metadata key. If you're using one of the built in patterns of reducing the values (e.g. aggregate, min, max) you can just say MetadataKey.aggregate<string>() instead of reimplementing the logic to reduce to an array of strings. You can see what this looks like for our predefined keys REQUIRED etc. Those define our built in metadata keys, but a user would have to do a similar thing to make their custom metadata keys

/gemini review