Expose internal registry usage metrics

As an OpenShift Image Team
I would like to be able to visualize the image registry usage metrics
So I can better identify users that heavily depend on the internal Image Registry
 
ACCEPTANCE CRITERIA

Has exposed the following metrics

MUST HAVE

• Number of puts and creates (blobs and manifests)
• Number of gets/serves (blobs and manifests) 
• Simple description of the new metrics (will become a KB article)

NICE TO HAVE

• Average size of served blobs directly
• Average size of pushed blobs
• Average size of pullthrough blobs
• Number of content offloads executed (direct access to the blob storage)
• Storage usage total size (nice to have)

NOTES

• We don't have the average size of the served blobs
• For storage usage size we may need a go routine that scans the whole storage
• The storage usage size may be a metric exposed by the operator instead