What are the main metrics you need to watch while running tests in order to do a proper performance tuning?

You need not see all of the 100 metrics your servers have. Webserver metrics, application server metrics & database server metrics. You can not judge many time by seeing all the metrics at the same time and getting confused your self and ending up with nothing to do and nothing to understand.
So you have to start from high end metrics like CPU utilization, Memory, Input/Output (I/O), Average load etc...
Understand the bottle necks; see the scalability of the environment.Look for other metrics only if you need additional information