Add custom kubefed metrics and expose them on a prometheus friendly format

[hectorj2f]

What would you like to be added:

I'd like to add custom metrics about kubefed that could be expose in a prometheus friendly basis analogously as other controller does in the Kubernetes ecosystem.

Why is this needed:

My intention would be to add custom metrics and a baseline that would enable to easily expose more metrics in the future.

/kind feature

[irajdeep]

@hectorj2f We recently started using kubefed. So as an exercise to get familiar with the codebase, I can work on this, if no one else is currently working on it.

[hectorj2f]

@irajdeep I already have a PR in the pipeline so I might create very soon. However I am willing to see if we can all benefit from your metrics as well.

[irajdeep]

@hectorj2f one of the metrics I had in mind related to how many objects kubefed failed to federate. If you haven't already implemented this metric, I can look into it.

[hectorj2f]

@irajdeep look at #1196 :)

[irajdeep]

@hectorj2f nice. So I think I can wait for your PR to be merged to work on top of it to expose metrics related to objects which couldn't be federated

[hectorj2f]

I am closing the issue, we added some custom metrics now