@Nullable
    default Set<TransientServiceOption> transientServiceOptions() {
        return null;
    }
Returns the {@link Set} of {@link TransientServiceOption}s that are enabled for a
{@link TransientService}.

<p>The default value of this flag is an empty string, which means all
{@link TransientServiceOption}s are disabled.
Specify the {@code -Dcom.linecorp.armeria.transientServiceOptions=<csv>} JVM option
to override the default value. For example,
{@code -Dcom.linecorp.armeria.transientServiceOptions=WITH_METRIC_COLLECTION,WITH_ACCESS_LOGGING}.
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java