Spring AI DefaultUsage#nativeUsage can crash SimpleLoggingAdvisor

The nativeUsage object within DefaultUsage may not be JSON serialisable and hence causes a crash when using SimpleLoggingAdvisor.

I've hit this when using VertexAI, where the nativeUsage instance, which is cast as an Object, is a protobuf of type GenerateContentRepsonse$UsageMetadata.

This can't be serialised for pretty printing within the SimpleLoggerAdvisor repsonse code, which calls ModelOptionsUtils#toJsonStringPrettyPrinter and thus throws an Exception.

    public static String toJsonStringPrettyPrinter(Object object) {
        try {
            return OBJECT_MAPPER.writerWithDefaultPrettyPrinter().writeValueAsString(object);
        }
        catch (JsonProcessingException e) {
            throw new RuntimeException(e);
        }
    }

I think simplest solution would be to add a @JsonIgnore to DefaultUsage#getNativeUsage as it's not always obvious if it can be logged.