### Question:
HttpTraceContext implements TextMapPropagator { @Override public List<String> fields() { return FIELDS; } private  HttpTraceContext(); static HttpTraceContext getInstance(); @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 fieldsList() { assertThat(httpTraceContext.fields()).containsExactly(TRACE_PARENT, TRACE_STATE); }