Support scrape Contour/Envoy metrics by Prometheus Operator

[izturn]

Please describe the problem you have

// TODO [danehans]: Remove the prometheus annotations when Contour is updated to
// show how the Prometheus Operator is used to scrape Contour/Envoy metrics.
PodAnnotations: map[string]string{
	"prometheus.io/scrape": "true",
	"prometheus.io/port":   "8002",
	"prometheus.io/path":   "/stats/prometheus",
},

// TODO [danehans]: Remove the prometheus annotations when Contour is updated to
// show how the Prometheus Operator is used to scrape Contour/Envoy metrics.
Annotations: map[string]string{
	"prometheus.io/scrape": "true",
	"prometheus.io/port":   fmt.Sprintf("%d", metricsPort),
},

I'd like to implement this as follows:

1. add a flag to ContourDeployment:

type ContourDeploymentSpec struct { // +optional EnableMetrics bool json:"enableMetrics,omitempty" }

2. if EnableMetrics equals to false, do nothing
3. otherwise, expose ports for services: contour/8000 & envoy/8002
4. kubectl apply CR\ServiceMontior by themself (optional)
5. that's all

@skriss @sunjayBhatia any advise?