code: public static SpanId fromLowerBase16(CharSequence src) {
      Utils.checkNotNull(src, "src");
      // TODO: Remove this extra condition.
      Utils.checkArgument(
          src.length() == BASE16_SIZE,
          "Invalid size: expected %s, got %s",
          BASE16_SIZE,
          src.length());
      return fromLowerBase16(src, 0);
    }
docs: Returns a {@code SpanId} built from a lowercase base16 representation.
  
  @param src the lowercase base16 representation.
  @return a {@code SpanId} built from a lowercase base16 representation.
  @throws NullPointerException if {@code src} is null.
  @throws IllegalArgumentException if {@code src.length} is not {@code 2 * SpanId.SIZE} OR if the
  {@code str} has invalid characters.
  @since 0.11