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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