Macro props 

macro_rules! props {
    () => { ... };
    ($($key:expr => $value:expr),+ $(,)?) => { ... };
}

Builds Props from key => value pairs.

use privatus_analytics::props;

let props = props! { "plan" => "pro", "role" => "admin", "seats" => 5 };
assert_eq!(props.len(), 3);
let none = props! {};
assert!(none.is_empty());