static String getKeyType() {
    return "type.googleapis.com/google.crypto.tink.HpkePrivateKey";
  }
Registers an {@link HpkePrivateKeyManager} and an {@link HpkePublicKeyManager} with the
registry, so that HpkePrivateKey and HpkePublicKey key types can be used with Tink.
java_src/src/main/java/com/google/crypto/tink/hybrid/internal/HpkePrivateKeyManager.java