Allow envoy StatsMatcher to be configurable

[davidgiga1993]

Envoy by default exposes tons of metrics, a lot of them are on a per-cluster basis, causing the number of metrics to increase with the number of ingress endpoints. This can cause issues with scaling Prometheus as well as unnecessary network traffic for scraping.

Other projects are facing similar issues (hashicorp/consul#12509)

Therefore I propose to allow the envoy StatsMatcher to be configurable so we can choose what metrics should be excluded directly at envoy.

[github-actions]

Hey @davidgiga1993! Thanks for opening your first issue. We appreciate your contribution and welcome you to our community! We are glad to have you here and to have your input on Contour. You can also join us on our mailing list and in our channel in the Kubernetes Slack Workspace