Design your metric formula.
Each product is different and requires a different set of metrics. There is no universal formula.
When you determine what metrics are truly important to you, you can create a formula in order to have one simple number. This is an indicator of your performance.
Create a formula that will represent your performance in growth.
Take this formula of eBay as an example: Key metric = number of sellers × number of listed items × number of buyers × number of successful transactions.