code: @Nullable
      public static String sampledToString(@Nullable Boolean sampled) {
          return sampled == null ? null : (sampled ? DO_SAMPLE_VALUE : DO_NOT_SAMPLE_VALUE);
      }
docs: Converts a sampled (Boolean) value to its String representation.
  <p/>
  Note: If this method is invoked with a null value, the returned value will also be null.
  
  @param sampled The original value
  @return The converted sampled value