I usually copy the stuff I already exposed in the UniFFI bindings and adapt it to the flutter_rust_bridge. So, in the case of metadata stuff, I would copy and adapt this:

GitHub

nostr/bindings/nostr-sdk-ffi/src/protocol/nips/nip01.rs at d7a8372e28a5dd7e1ce02b725ee0b43c798e332f · rust-nostr/nostr

Rust implementation of the nostr protocol, high-level client library, Nostr Wallet Connect and more. - rust-nostr/nostr

To adapt it, I follow these docs

Introduction | flutter_rust_bridge

Rust Package

or I just check the other modules I already adapted. In short: replace the uniffi macros with `frb` and use the `Result` from anyhow. If a new module is added, it must be exported in the `lib/nostr_sdk.dart` file.