[doc] 4.3 Stratos Metrics corrections

[cduch]

4.3 Stratos Metrics

https://www.suse.com/documentation/cloud-application-platform-1/singlehtml/book_cap_guides/book_cap_guides.html#sec.cap.stratos-metrics

1. If someone does not define a default storage class, the deployment will fail if followed by these instructions. Please change the stratos-metrics-values.yaml example to include the parameter for the storageClass for Prometheus (the inherited helm chart).
2. If SUSE Enterprise Storage is used, the ceph-admin key also needs to be copied to the "metrics" namespace to get it working.
3. I am also not 100% sure if it's additionally required to give the serviceaccount "default" the psp clusterrolebinding for the namespace "metrics".

[btat]

@nwmac Do you know if point 3 above is required?