Add velero_backup_active_total metric

Describe the problem/challenge you have Currently it is hard to see how many backup jobs are running.

Describe the solution you'd like Expose a velero_backup_active_total metric. An integer value starting at 0.

Anything else you would like to add: Maybe the pods running the actual backup could report their progress too?

Environment:

• Velero version (use velero version): v1.2.0
• Kubernetes version (use kubectl version): v1.15.6
• Kubernetes installer & version: v1.15.3
• Cloud provider or hardware configuration: on-premise
• OS (e.g. from /etc/os-release): Ubuntu 18.04