Function binary_metric_name_side 

fn binary_metric_name_side<'a>(
    lhs: &'a Expr,
    rhs: &'a Expr,
    modifier: Option<&BinModifier>,
) -> &'a Expr

The operand whose labels supply the result metric name of a binary operation.

Prometheus builds the result from the side whose labels it carries, i.e. the “many” side of the match: group_right makes the right-hand operand that side, and a scalar left-hand operand (for example 0.5 < a) leaves the vector on the right. Every other case takes the left-hand operand.