Macro describe_histogram 

macro_rules! describe_histogram {
    ($name:expr, $unit:expr, $description:expr $(,)?) => { ... };
    ($name:expr, $description:expr $(,)?) => { ... };
}

Describes a histogram.

Histograms measure the distribution of values for a given set of measurements, and start with no initial values.

Histograms can be described with a free-form string, and optionally, a unit can be provided to describe the value and/or rate of the measurements. Whether or not the installed recorder does anything with the description, or optional unit, is implementation defined.

Example

// A basic histogram:
describe_histogram!("some_metric_name", "my favorite histogram");

// Providing a unit for a histogram:
describe_histogram!("some_metric_name", Unit::Bytes, "my favorite histogram");

// As mentioned in the documentation, metric names also can be owned strings, including ones
// generated at the callsite via things like `format!`:
let name = String::from("some_owned_metric_name");
describe_histogram!(name, "my favorite histogram");

describe_histogram!(format!("{}_via_format", "name"), "my favorite histogram");