Macro request_header

macro_rules! request_header {
    ($(#[$outer:meta])* $name:ident, $header:ident) => { ... };
    ($(#[$outer:meta])* $name:ident, $header:ident, $(($variant:ident, $value:expr)), *) => { ... };
}

The following macro invocation:

request_header!(
    /// Builds a client request id header
    ClientRequestId, CLIENT_REQUEST_ID,
);

Turns into a Header value used to construct requests.