Just to be clear this functionality was already present but the documentation for it was commented out?

It's not just the documentation. It seems that, this feature was included as part of the initial filter BUT, is not working. With this PR i'm reenabling the TokenRealy filter through properties, and uncomment the related documentation.

Could you add a test to confirm it is now working?

I'm sorry, but i don't see an example to test filter functions through properties in gateway-server-mvc to take as base, and my knowledge is limited at this point.

A manual test with properties like these:

spring:
  cloud:
    gateway:
      mvc:
        routes:
          - id: token_relay_test
            uri: https://examplel1.com
            filters:
              - TokenRelay=relay
  security:
    oauth2:
      client:
        provider:
          relay:
            jwk-set-uri: https://localhost:8080/context/path/jwk
            issuer-uri: https://localhost:8080/context/path/issuer
        registration:
          relay:
            provider: relay
            client-authentication-method: client_secret_post
            authorization-grant-type: client_credentials
            client-id: someClientId
            client-secret: someClientSecret

works perfectly using it in a complete spring boot application.

If someone could create a specific test for this, I would be grateful.

@jaimesf I have spent some time and added a test to your PR to test this, see this class https://github.com/spring-cloud/spring-cloud-gateway/pull/3751/files#diff-b6a9d0f37208e4836a1687c1714f1ea8058daca82bd634b88237aef65c70bdf8

Unfortunately to do this I had to make several other changes because Spring Security is on the classpath. In addition once Spring Security was on the classpath it revealed a completely unrelated bug which I documented here #3816