Macro prometheus_utils::label_enum

macro_rules! label_enum {
    ($(#[$attr : meta]) * enum $N : ident
 { $($(#[$var_attr : meta]) * $V : ident), * $(,) * }) => { ... };
    ($(#[$attr : meta]) * pub enum $N : ident
 { $($(#[$var_attr : meta]) * $V : ident), * $(,) * }) => { ... };
    ($(#[$attr : meta]) * pub($($vis : tt) +) enum $N : ident
 { $($(#[$var_attr : meta]) * $V : ident), * $(,) * }) => { ... };
}

Declare an enum intended to be used as a Prometheus label.

This helper macro can only be used to define enums consisting of tags without values. Each tag corresponds to a possible Prometheus label value. The macro then generates two functions, as_str and all_variants, as described in the example below. Those funtions are intended to assist in implementing the Labels trait for a label struct that contains the enum, ensuring a consistent conversion to strings for label values, and that all possible variants are included when implementing possible_label_values.

Example

When using the macro, define exactly one enum inside, as you would normally:

label_enum! {
    pub(crate) enum MyErrorLabel {
        IoError,
        TimeoutError,
        MemoryError,
    }
}

The macro will declare the enum exactly as provided. But in addition, it will generate two functions:

impl MyErrorLabel {
    /// The name of this enum variant, as a string slice.
    pub fn as_str(&self) -> &'static str { ... }

    /// A vector containing one instance of each of the enum's variants.
    pub fn all_variants() -> Vec<Self> { ... }
}