feat(metrics): add machine deployment metrics

[rajaSahil]

What this PR does / why we need it:

• add machine deployment metrics

# HELP machine_deployment_available_replicas The number of available replicas for a machine deployment
# TYPE machine_deployment_available_replicas gauge
machine_deployment_available_replicas{name="",namespace=""} 0
# HELP machine_deployment_ready_replicas The number of ready replicas for a machine deployment
# TYPE machine_deployment_ready_replicas gauge
machine_deployment_ready_replicas{name="",namespace=""} 0
# HELP machine_deployment_replicas The number of replicas defined for a machine deployment
# TYPE machine_deployment_replicas gauge
machine_deployment_replicas{name="",namespace=""} 0
# HELP machine_deployment_updated_replicas The number of replicas updated for a machine deployment
# TYPE machine_deployment_updated_replicas gauge
machine_deployment_updated_replicas{name="",namespace=""} 0

Which issue(s) this PR fixes:

Fixes #1767

What type of PR is this?
/kind feature

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

 Add collector to expose machine deployment metrics

Documentation:

NONE

[rajaSahil]

/assign @embik

[embik]

@rajaSahil did you see the linter errors?

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[rajaSahil]

@embik Please re-review, have fixed lint errors.

[embik]

I suspect this is something you did because of linting errors, can we please just quickly document (in this thread) why it's being removed?

[rajaSahil]

Linter threw this error pkg/cloudprovider/provider/anexia/types/types.go:45:5: the sentinel error name StatusUpdateFailedshould conform to theErrXxx format (errname) and it is not getting used anywhere that's why I removed it.