Metadata as first class

[timonv]

Is your feature request related to a problem? Please describe.
Medatadata is currently a map with strings to strings. It would be nice to be able to programmatically use metadata.

Describe the solution you'd like
Similar to prompts, with a Metadata object that takes a Value, where value can be different primitives.

A good example for intended use; if we implement a knowledge graph, a vector of keywords would make excellent edges.