Add Ready replicas metric for deployments

[akshitgrover]

What would you like to be added:
Ready Replicas count for the deployment object is missing from the list and is absent from the source code as well
I'd like this to be reported like all other metrics for deployments eg: desired, available replicas etc.

Why is this needed:
Ready Replicas count is an insightful metric as it depicts the number of traffic serving pods for a particular deployment, This can be used in number of ways to compare across the board for troubleshooting

Describe the solution you'd like
The solution is quite simple, We'll just need to add one more metric generator inside 'deploymentMetricFamilies' method using the deployment.Status.ReadyReplicas field

Additional context
None

[akshitgrover]

I'd like to work on this as well
I hope it's fine @mrueg @brancz
/assign

[akshitgrover]

Does this need triage like issues in Kubernetes main repo?
cc @mrueg