Measure service unavailability during upgrade
What you suggest might be the simplest way to collect the data, but you will have to do quite a bit of work to extract the availability over certain periods. 

I think it's fair to say that if you want availability, you need a monitoring system. This means having an extra service in your catalogue to continuously probe the availability of your microservices over time. Storing them in a time-series database would allow you to make queries to establish availability over various periods.

There are many tools which could do this for you. A good starting place would be the CNCF monitoring landscape