Zone abstraction of object layer was returning `nil`
incorrectly under situations where disk healing is
not required. Returning `nil` is considered as healing
successful, which leads to unexpected ReloadFormat()
peer notification calls during startup.
This PR fixes this behavior properly for zones.
Admin data usage info API returns the following
(Only FS & XL, for now)
- Number of buckets
- Number of objects
- The total size of objects
- Objects histogram
- Bucket sizes
This PR adds support below metrics
- Cache Hit Count
- Cache Miss Count
- Data served from Cache (in Bytes)
- Bytes received from AWS S3
- Bytes sent to AWS S3
- Number of requests sent to AWS S3
Fixes#8549
