Integrate Consul Connect with Flagger

I submitted a pull request for this feature in Flagger repo: https://github.com/weaveworks/flagger/pull/482

Here I would like to integrate it further in terms of samples, docs, required Prometheus scraper configuration.

#### Feature Description

[Flagger](https://github.com/weaveworks/flagger) is a Kubernetes operator that is providing declarative control over various deployment strategies for service meshes. It's great to have Flagger integrated with Consul Connect.

#### Use Case(s)

- doing canaries, A/B deployments, blue/green
- simple declarative control over deployments