Metrics rest endpoint

[aphralG]

Proposed changes

Add REST endpoints for metrics.

• Added prometheus exporter extension
• Moved metric calculation map from aggregate.go to metrics_util.go

To Do

• Unit Tests
• Component Tests

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md)

[Dean-Coakley]

Suggested change

	} else {
	return prometheus.GaugeValue
	}
	}