DefaultUsage#nativeUsage can crash SimpleLoggingAdvisor

[mands]

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.