Prometheus 500 Internal Error on Spring Actuator

The root cause is your customized HttpMessageConverter stopped processing of WebSecurity

@EnableWebMvc
@Configuration
public class WebConfig implements WebMvcConfigurer {

    @Override
    public void addCorsMappings(CorsRegistry registry) {
        registry.addMapping("/**")
                .allowedOrigins("*")
                .allowedMethods("HEAD", "GET", "PUT", "POST", "DELETE", "PATCH");
    }

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        final Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder()
                .indentOutput(true).dateFormat(new SimpleDateFormat("yyyy-MM-dd"));

        converters.add(new MappingJackson2HttpMessageConverter(builder.build()));
    }
}