Refactor Registry::new_custom() to accept AsRef<str> instead of String for improved flexibility

[j3rrywan9]

Refactor Registry::new_custom() to accept AsRef<str> instead of String for improved flexibility, so that the labels! macro can be used for the 2nd parameter here. For example:

let registry = Registry::new_custom(Some("test"), Some(labels!{"environment" => "test"})).unwrap();