Function method_descriptor 

pub fn method_descriptor<'a, 'r, A: Facet<'a>, R: Facet<'r>>(
    service_name: &'static str,
    method_name: &'static str,
    arg_names: &[&'static str],
    doc: Option<&'static str>,
) -> &'static MethodDescriptor

Build and leak a MethodDescriptor with default volatile retry policy.