Web Application Server Monitoring

I've seen several StackOverflow posts that discuss what tools to use to monitor web application performance, but none that talk about what metrics to focus on.

What web server metrics should be monitored and which should have alerts setup on?

Here are some I currently have in mind:

• requests timeouts (alerts)
• requests queued (alerts)
• time to first byte (may need to be monitored externally)
• requests / second

Also, how can these be measured on a java web application server.