@Override
  public Optional<String> getKid() {
    return kid;
  }
Returns the "kid" to be used for this key.

<p>If present, this kid will be written into the {@code kid} header during {@code
computeMacAndEncode}. If absent, no kid will be written.

<p>If present, and the {@code kid} header is present, the contents of the {@code kid} header
needs to match the return value of this function.
java_src/src/main/java/com/google/crypto/tink/jwt/JwtHmacKey.java