How to edit the Principal's role and name information in a Spring WebFlux application running as OAuth2 Resource Server?

Question

I'm developing an application where my back-end is a stateless resource server. Every request that it receives should contain an Authentication header with a JWT Bearer token - currently a Google id_token. I'm able to verify the token, but in order to achieve method level security I need to be able to set the principal's role and name with information from a repository/service. How can I set the role and name of the principal?

Following the Spring tutorial, I'm trying to define a UserDetailsService bean to set the information, but it doesn't seem to be invoked. I've added a breakpoint in the findByUsername method but the execution never stops there.

application.yml file

spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          issuer-uri: https://accounts.google.com

Config.java file:

@EnableWebFluxSecurity
@EnableReactiveMethodSecurity
public class SecurityConfig {

    @Bean
    SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) throws Exception {
        http
                .authorizeExchange()
                .pathMatchers("/**")
                .authenticated()
        .and()
                .oauth2ResourceServer()
                .jwt();
        return http.build();
    }

    // Using MapReactiveUserDetailsService, but I'll create my custom UserDetailsService implementation
    @Bean
    public MapReactiveUserDetailsService userDetailsService() {
        User.UserBuilder userBuilder = User.withDefaultPasswordEncoder();
        UserDetails rob = userBuilder.username("rob") //Instead of 'rob' and 'admin' I have in my code the Google ID of the Google accounts that I'm using to create the token
                .password("rob")
                .roles("USER")
                .build();
        UserDetails admin = userBuilder.username("admin") 
                .password("admin")
                .roles("USER","ADMIN")
                .build();
        return new MapReactiveUserDetailsService(rob, admin);
    }

SecuredController.java file:

@RestController
public class SecuredController {
    @GetMapping("/secured")
    @PreAuthorize("hasRole('ADMIN')")
    public Mono<Principal> secured(Principal principal) {
        return Mono.just(principal);
    }
}

With this configuration the user details service is not being called and I can't reach the GET /secured endpoint. I would like to reach it using the user with ADMIN role.