Counter and Family encode empty LabelSet differently

If you create a Family, the empty label set will encode like metric{} 2. If you use a raw Counter, it will encode like counter 2.

This seems like an unintended difference

use std::fmt::Error;
use prometheus_client::encoding::EncodeLabelSet;
use prometheus_client::encoding::text::encode;
use prometheus_client::metrics::counter::Counter;
use prometheus_client::metrics::family::Family;
use prometheus_client::registry::Registry;

#[test]
fn test_counter_vs_family() {
    let mut registry = Registry::default();
    let family: Family::<EmptyLabels, Counter> = Family::default();
    let ctr: Counter = Counter::default();
    registry.register("family", "", family.clone());
    registry.register("counter", "", ctr.clone());

    #[derive(Eq, PartialEq, Hash, Debug, Clone)]
    struct EmptyLabels {}

    impl EncodeLabelSet for EmptyLabels {
        fn encode(&self, _encoder: &mut prometheus_client::encoding::LabelSetEncoder) -> Result<(), Error> {
            Ok(())
        }
    }

    family.get_or_create(&EmptyLabels {}).inc();
    ctr.inc();

    let mut encoded = String::new();

    encode(&mut encoded, &registry).unwrap();
    println!("{}", encoded);
}

# HELP family .
# TYPE family counter
family_total{} 1
# HELP counter .
# TYPE counter counter
counter_total 1
# EOF

Start with the provided test using prometheus_client::encoding::text::encode and compare the Counter and Family outputs shown in the issue. Trace the encoding entry points for Counter and Family, then confirm that the completed work makes empty label sets encode consistently and that the test passes.