The critical issues are always cost, schedule, and performance (quality) objectives. The PM has to analyze critical items, allow risk to each, and then monitor the variance to risk (in cost, schedule, and performance). What metrics? In PERT, the critical path is well known, measurements on that are very accurate. It sets completion dates, material due dates, and impacts. All those are easy to monitor. Cost is easy to monitor and project, given material costs and schedule performance. Quality or Performance measures are more difficult in technical areas, because often the alligators don't surface until the testing phase at the end. Howver, risk can be assigned to these, and risk can be translated to cost and schedule impact.
Summary: Measure schedule performance, cost to date and projected cost, and performance (quality and technical objectives). Having a honest progressman on the job is a big help.
That's what I was thinking. I just wasn't sure I understood the question correctly. Thank you very much.