Rainbow table doesn't support IDs of type long

We have a legacy db with very long Ids. Example: 301000000003

So the POCO looks like this:

public class User
{
    public long Id { get; set; }
    // ...
}

And the database class like this:

public class LegacyDatabase : Database<LegacyDatabase>
{
    public Table<User, long> Users { get; set; }
    // ...
}

When calling InsertAsync however there is an exception thrown.
This is because of the fixed return type of int? for InsertAsync. See: Database.Async.cs#L19

Is this by design? Shouldn't the generic type TId be used here?

Start in Dapper.Rainbow/Database.Async.cs at line 19 and compare the InsertAsync signature with the Table<User, long> usage described in the issue. Reproduce the long-ID InsertAsync case, inspect related Rainbow APIs and tests, and consider the work complete when insertion succeeds and the returned ID supports the declared TId type.