Make `Metric::measurement_name()` not take `&self`, if possible

[theodorebugnet]

Consider

pub struct MetricBatch<M: Metric> {
    pub metrics: Vec<M>,
}

impl<M: Metric> Metric for MetricBatch<M> {
    fn measurement_name(&self) -> &'static str {
        // What here?
    }
    //...
}

In practice since the method returns &'static str, it effectively never depends on &self. If the function is refactored to not take self, then the batch implementation could simply call M::measurement_name().