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

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[rajaSahil]

/assign @embik

[kubermatic-bot]

@rajaSahil: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-machine-controller-lint	bc715cd	link	true	/test pull-machine-controller-lint

Full PR test history

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[embik]

@rajaSahil did you see the linter errors?