static int fieldNumber(int key, int byteL) {
      int fieldNumber = key >>> 3;
      if (fieldNumber != 0) return fieldNumber;
      throw new IllegalArgumentException("Malformed: fieldNumber was zero at byte " + byteL);
    }
"Each key in the streamed message is a varint with the value {@code (field_number << 3) |
wire_type}"

<p>See https://developers.google.com/protocol-buffers/docs/encoding#structure
zipkin/src/main/java/zipkin2/internal/Proto3Fields.java