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?

[rajaSahil]

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

[embik]

/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 394dad768e8cd55e713a510c3c533fd08e541c5a

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: embik

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [embik]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment