Follow-up to CSHARP-5693, which added MongoClient.AppendMetadata(LibraryInfo) to let an already-instantiated client send client handshake metadata on-demand.

That method was added on the concrete MongoClient class only, not on the IMongoClient interface. Adding a member to IMongoClient is a breaking change for implementers (mocks, test doubles, custom wrappers), and the driver multi-targets net472, which cannot consume default interface methods — so the interface addition must wait for a major version bump.

Acceptance criteria

• Add void AppendMetadata(LibraryInfo libraryInfo) to IMongoClient.
• Update all in-codebase implementers accordingly.
• Target the 4.0.0 release.