code: public static StubTransformer fixedCredentialsStubTransformer(final CallCredentials credentials) {
          requireNonNull(credentials, "credentials");
          return (name, stub) -> stub.withCallCredentials(credentials);
      }
docs: Creates a new {@link StubTransformer} that will assign the given credentials to the given {@link AbstractStub}.
  
  @param credentials The call credentials to assign.
  @return The transformed stub.
  @see AbstractStub#withCallCredentials(CallCredentials)