code: @Override public <C> BraveSpanContext extract(Format<C> format, C carrier) {
      Extractor<TextMap> extractor = formatToExtractor.get(format);
      if (extractor == null) {
        throw new UnsupportedOperationException(format + " not in " + formatToExtractor.keySet());
      }
      TraceContextOrSamplingFlags extractionResult = extractor.extract((TextMap) carrier);
      return BraveSpanContext.create(extractionResult);
    }
docs: Extracts the underlying context using B3 encoding by default. Null is returned when there is no
  encoded context in the carrier, or upon error extracting it.