Is your feature request related to a problem? Please describe.
Tools currently support annotations (which can be predefined, but additional annotations can be added), and resources support metadata.

However, Prompts do not have any way to send additional data, for example "tag" them somehow.

An example use case is when you want to send a list of Prompts to the MCP client, and the client will then use the data to render them to the end user in the UI, for example prompts can be categorized via metadata, otherwise it's just a big list that the UI would receive.

Describe the solution you'd like
Add metadata/annotations to prompts similar to tools/resources.

Describe alternatives you've considered
Sending a json-encoded string in the prompt description is a bad workaround and not a real solution. With the current prompt arguments, there is no way to send extra metadata from the server to the client.