Minimal APIs currently don't populate any value for the ApiDescriptor.GroupName property. We should add support for setting the group name via endpoint metadata and flowing that through to the ApiDescriptor, e.g.:

app.MapGet("/myapi/do-the-thing", () => "done!")
   .WithGroupName("v1");

// Psuedo-spec
public static IEndpointConventionBuilder WithGroupName(this IEndpointConventionBuilder builder, string groupName)
{
    builder.WithMetadata(new EndpointGroupNameMetadata(groupName));
}

// Attribute for declarative scenario
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Delegate, Inherited = false, AllowMultiple = false)]
public sealed class EndpointGroupNameAttribute: Attribute, IEndpointGroupNameMetadata;

Setting it by default
Swashbuckle currently uses ApiDescriptor.GroupName in its default API inclusion predicate in such a way that if we were to set a default value that doesn't match the configured Swagger document name, the APIs would not be included by default. For that reason it seems like we shouldn't set a default value for ApiDescriptor.GroupName unless we also update the Swashbuckle logic.

@bradygaster