Expanding Spring Modulith tracing tags

Perhaps you use reflection to avoid copy/pasta?

@Bean
ModulithObservationConvention modulithObservationConvention() throws Exception {
    Class<?> type = Class.forName("org.springframework.modulith.observability.support.DefaultModulithObservationConvention");
    type.setAccessible(true);
    Constructor constructor = type.getDeclaredConstructor();
    constructor.setAccessible(true);
    ModulithObservationConvention delegate = (ModulithObservationConvention) constructor.newInstance();
    return new ModulithObservationConvention {
        @Override
        public boolean supportsContext(Observation.Context context) {
            return delegate.supportsContext(context);
        }

        @Override
        public String getName() {
            return delegate.getName();
        }

        @Override
        public KeyValues getLowCardinalityKeyValues(ModulithObservationContext context) {
            return delegate.getLowCardinalityKeyValues(context);
        }

        @Override
        public KeyValues getHighCardinalityKeyValues(ModulithObservationContext context) {
            KeyValues delegateKvs = delegate.getHighCardinalityKeyValues(context);
            KeyValues customKvs = KeyValues.of(
                "bean.name", "sample",
                "class.name", "foo.bar.Sample"
            );
            return KeyValues.concat(delegateKvs, customKvs); 
        }
    };
}

You can’t extend DefaultModulithObservationConvention, it’s package-private by design. The proper way is to define your own ObservationConvention<ModulithObservationContext> bean. Spring Modulith will automatically use it instead of the internal default.

@Bean
ObservationConvention<ModulithObservationContext> modulithObservationConvention() {
    return new ObservationConvention<>() {
        @Override
        public boolean supportsContext(Observation.Context context) {
            return context instanceof ModulithObservationContext;
        }

        @Override
        public String getName() {
            return "modulith.observation";
        }

        @Override
        public KeyValues getLowCardinalityKeyValues(ModulithObservationContext context) {
            return KeyValues.of(
                "module.key", context.getModule().getName(),
                "module.method", context.getMethod().getName()
            );
        }

        @Override
        public KeyValues getHighCardinalityKeyValues(ModulithObservationContext context) {
            return KeyValues.of(
                "bean.name", context.getBean().getClass().getSimpleName(),
                "class.name", context.getMethod().getDeclaringClass().getName()
            );
        }
    };
}

This replaces the built-in convention and lets you add any extra tags (bean.name, class.name, etc.) No internal classes need to be copied or modified.

In short: register your own ObservationConvention<ModulithObservationContext> bean : it safely overrides the default and gives you full control over trace tags.