Document auth changes to metrics endpoints from PE-28647 and PE-28468

Puppet Version: 5.5
Puppet Server Version: 5.3
OS Name/Version:

I had some feedback from a user that said the following docs page doesn't reflect the changes made in the latest Server version. They didn't give any more information than that though. 

https://puppet.com/docs/puppetserver/5.3/metrics-api/v1/metrics_api.html

But please close if this page looks right to you. Thanks!

Changes needed:

• the /metrics/v1 endpiont is now disabled by default
• both the v1 and the v2 endpoints now require a certificate to be sent when querying them