`Runtime` could provide a way to register with the current runtime

[cratelyn]

this would be a small tweak for ease of use.

currently, one must explicitly provide a runtime handle, e.g.:

        let registry = registry.sub_registry_with_prefix("tokio_rt");
        let runtime = tokio::runtime::Handle::current();
        let metrics = kubert_prometheus_tokio::Runtime::register(registry, runtime);

it would be nice if there was a way to use the current runtime by default.