what metric should I monitor for a spider in scrapy
Your question doesn't compute, because the answer depends solely on your specific use case and the goal of your scraping. The criteria/metric of spiders success is directly derived from your business logic. Just take a step back and ask yourself, what do I want to achieve with this spider? When you answer that, the metric you should monitor will be very clear.
To make sure everything is working properly, what metric should I monitor for a general spider?
Currently, I'll monitor

the number of item returned. E.g. the percentage change should be less than 1%
the number of error in log file. E.g. the error number should be zero

is it enough for a general spider?