Some examples of Project Metrics would be -
Schedule & Effort Metrics such as Planned Effort & Actual Effort, Planned Start Date and Actual Start Date.
Cost Metrics could include Cost of Quality , Cost Variance
Other metrics could include CPI , SPI and other variables used in EVM
You can also have Risk Metrics such as risk indicators , probability of resource availability etc
The idea of having project metrics is to track & improve project performance. However, collecting project metrics is a time consuming task so you should choose what you need to track judiciously.
A few key metrics that can be used could include schedule slippage in days, cost variance, effort variance, number of defects