code: public GrpcChannelBuilder setCredentials(String userName, String password,
        String impersonationUser) {
      mUserName = userName;
      mPassword = password;
      mImpersonationUser = impersonationUser;
      mUseSubject = false;
      return this;
    }
docs: Sets authentication content. Calling this will reset the subject set by {@link #setSubject}.
  
  @param userName the username
  @param password the password
  @param impersonationUser the impersonation user
  @return the updated {@link GrpcChannelBuilder} instance