Add descriptions to the metrics documentation

**Is your feature request related to a problem? Please describe.**
Currently it's not really clear when viewing the metrics for a Static Web App what the different metrics represent (e.g. SiteHits, CdnRequestCount).

**Describe the solution you'd like**
There is already a documentation page that lists some information about the different metrics available in a Static Web App [here](https://docs.microsoft.com/en-us/azure/azure-monitor/essentials/metrics-supported#microsoftwebstaticsites), but there are no descriptions.
It would be great if someone who knows what each metric means could update that documentation.

**Describe alternatives you've considered**
N/A

**Additional context**
N/A