@Nullable public String getValue(@Nullable TraceContext context) {
    if (context == null) return null;
    return this.context.getValue(this, context);
  }
@Test void getValue_context_doesntExist() {
    assertThat(AMZN_TRACE_ID.getValue(requestIdContext))
      .isNull();
    assertThat(AMZN_TRACE_ID.getValue(emptyContext))
      .isNull();
    assertThat(AMZN_TRACE_ID.getValue(context))
      .isNull();
  }