public static Function<? super HttpClient, ObservationClient> newDecorator(
            ObservationRegistry observationRegistry) {
        requireNonNull(observationRegistry, "observationRegistry");
        return delegate -> new ObservationClient(delegate, observationRegistry, null);
    }
Creates a new micrometer observation integrated {@link HttpClient} decorator
using the specified {@link ObservationRegistry}.
core/src/main/java/com/linecorp/armeria/client/observation/ObservationClient.java