Add a metric of AppWrappers counts per state

[ronensc]

Issue link

Closes #674

What changes have been made

This PR adds a custom metric to expose how many AppWrappers are per state.
It is built on top of #651. To review only the delta, skip the first 3 commits in this PR.

Verification steps

Same as mentioned in #651, I checked that the metrics are available in the codeflare-operator operator by running it:

# in codeflare-operator root
go mod edit -replace github.com/project-codeflare/multi-cluster-app-dispatcher=../multi-cluster-app-dispatcher
make build
NAMESPACE=default go run ./main.go -kubeconfig <path-to-kubeconfig>

and in a new shell:

curl localhost:8080/metrics | grep appwrappers_count


# HELP mcad_appwrappers_count AppWrappers count per state
# TYPE mcad_appwrappers_count gauge
mcad_appwrappers_count{state="Completed"} 0
mcad_appwrappers_count{state="Deleted"} 0
mcad_appwrappers_count{state="Failed"} 0
mcad_appwrappers_count{state="Pending"} 0
mcad_appwrappers_count{state="Running"} 1
mcad_appwrappers_count{state="RunningHoldCompletion"} 0

To see the counts change, I created a sample AppWrapper:

# in MCAD root
kubectl apply -f doc/usage/aw-01.yaml

Checks

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • Testing is not required for this change

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[ronensc]

In case of error, should we update the counters to 0 or leave them as is?

[mamy-CS]

I think we should leave them as is and retry to update with new value.

[ronensc]

This update is done periodically. So instead of retrying, we can simply wait for the next update on the next tick.
I dug deeper into the implementation of controller.appWrapperLister.List(labels.Everything()), and it seems that since labels.Everything().Empty() returns true, there is no way that controller.appWrapperLister.List() will return an error. So the discussion seems to be purely theoretical.