You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: counter.rs
    path of file: ./repos/cargo/src/cargo/util
    
    the code of the file until where you have to start completion: use std::time::Instant;

/// A metrics counter storing only latest `N` records.
pub struct MetricsCounter<const N: usize> {
    /// Slots to store metrics.
    slots: [(usize, Instant); N],
    /// The slot of the oldest record.
    /// Also the next slot to store the new record.
    index: usize,
}

imp
    
    fn counter() {
        let now = Instant::now();
        let mut counter = MetricsCounter::<3>::new(0, now);
        assert_eq!(counter.rate(), 0f32);
        counter.add(1, now + Duration::from_secs(1));
        assert_eq!(counter.rate(), 1f32);
        counter.add(4, now + Duration::from_secs(2));
        assert_eq!(counter.rate(), 2f32);
        counter.add(7, now + Duration::from_secs(3));
        assert_eq!(counter.rate(), 3f32);
        counter.add(12, now + Duration::from_secs(4));
        assert_eq!(counter.rate(), 4f32);
    }