macro_rules! matches syntax and expands to more syntax, before type checking. It does what a function cannot:

• take a variable number of arguments
• accept arguments of different types in the same position
• capture the source text of an expression (this is how assert_eq! prints both sides)

macro_rules! metric {
    ($name:expr, $value:expr) => { format!("{}={}", $name, $value) };
}