### Question:
TraceMultiPropagator implements TextMapPropagator { @Override public List<String> fields() { return propagatorsFields; } private  TraceMultiPropagator(List<TextMapPropagator> propagatorList); static Builder builder(); @Override List<String> fields(); @Override void inject(Context context, C carrier, Setter<C> setter); @Override Context extract(Context context, C carrier, Getter<C> getter);  }### Answer:
@Test void fields() { TextMapPropagator prop = TraceMultiPropagator.builder() .addPropagator(new EmptyPropagator("foo", "bar")) .addPropagator(new EmptyPropagator("hello", "world")) .build(); List<String> fields = prop.fields(); assertThat(fields).containsExactly("foo", "bar", "hello", "world"); }