Macro mandatory_doc 

macro_rules! mandatory_doc {
    ($doc:expr) => { ... };
    ($($doc:expr)+) => { ... };
    () => { ... };
}

This macro matches a series of doc comment attributes. Multi-line doc comments appear as a sequence of doc comment attributes, so we need to be able to match both the individual case and the sequence, and map them back to a single string.

While this macro accepts the no-doc-comment case, we require that every metric have a doc comment, so we will emit a compile error if none were present.