I'm a bit green to web applications although I am in the final phases of developing one for a client. I'm using Django with Gunicorn/Nginx on an AWS m1.medium. The database (MongoDB) is on a separate instance. The client is paranoid about scaling and so I threw together a crude monitoring server which has the ability to spin up new AWS instances, install the app code and load balance (I know, I know, I could have used ELB. I said I was green, and it was fun writing it anyway).
The question is, I don't really know what metrics I should be aware of. How will I know when my app server is under 'high load'? CPU? RAM? Request latency? All of the above?
Any guidance in this area would be appreciated.