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I still have a few open questions:

1. Do we prefer the name metadata or property for this concept?

2. What should be the names of the functions used to bind logic to MetadataKey and AggregateMetadataKey.
   Some ideas for MetadataKey include:

   • metadata (or property)
   • setMetadata (or setProperty)

   Some ideas for AggregateMetadataKey include:

   • addMetadata (or addProperty)
   • addToMetadata (or addToProperty)
   • aggregateMetadata (or aggregateProperty)
   • accumulateMetadata (or accumulateProperty)
   • contributeMetadata (or contributeToProperty)

3. It can be tedious to type the generic MetadataKey.create<...>() for a key that you use internally to the schema. I'm worried people will just yolo and MetadataKey.create<any>(), should we add back an equivalent to the 2-parameter variant of define?