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What data should I be capturing when monitoring my web service?

I have a web service that serves a front end web application. I log the duration of the http requests in ElasticSearch/Kibana. I also capture exceptions. I have the ability to add monitoring to Grafana by sourcing this data but am not sure what data I should be monitoring. In the past I've monitored for what I mentioned above. Do people have suggestions/best practices for monitoring?