Make the structure name inclusion in module optional

[gagbo]

I've always thought that the class/struct would just be part of the module, as in the end the name of a class acts as an additional scope for the name of the function.

I didn't even think of asking what other points of view were, I thought Rust was always doing that, so I actually had that implemented in the helper that lists all the the functions that have autometrics annotations:

#[test]
fn detect_trait_impl_block() {
    let source = r#"
        struct Foo{};

        #[autometrics]
        impl A for Foo {
            fn m_a() {}
        }
        "#;

    let list = list_function_names(String::new(), source, true).unwrap();

    assert_eq!(list.len(), 1);
    assert_eq!(
        list[0],
        ExpectedAmLabel {
            module: "Foo".into(),
            function: "m_a".into()
        }
    );
}

Originally posted by @gagbo in autometrics-dev/autometrics-shared#33 (comment)

[gagbo]

Turns out module_path! is used in autometrics-rs, so the information is not available.

It could be useful later though (at least to have a better list in diff-metrics related output), so instead of removing the code, we should make the struct inclusion a flag