public Set<TransientServiceOption> build() {
        if (transientServiceOptions == null) {
            return Flags.transientServiceOptions();
        }
        return Sets.immutableEnumSet(transientServiceOptions);
    }
Returns the {@link Set} of {@link TransientServiceOption}s set so far.
{@link Flags#transientServiceOptions()} if not set.
core/src/main/java/com/linecorp/armeria/internal/server/TransientServiceOptionsBuilder.java