Backport of Allow HCP metrics collection for Envoy proxies into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #16511 to be assessed for backporting due to the inclusion of the label backport/1.15.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

merge conflict error: POST https://api.github.com/repos/hashicorp/consul/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

---

Description

Add a new envoy flag: envoy_hcp_metrics_bind_socket_dir, a directory where a unix socket will be created with the name <namespace>_<proxy_id>.sock to forward Envoy metrics.
If set, this will configure a local stats_sink and STATIC cluster to forward Envoy metrics to an HCP collector.

The HCP Cloud team is working on enabling observability metrics in HCP.

There will be a follow-up PR to generate a listener and cluster dynamically that will be listening on this unix socket.
That listener and cluster will be configured to route to collector instances through the service mesh.

Caveat: we needed to add this local listener indirection as a workaround due to this issue

Testing & Reproduction steps

• Added unit tests
• Tested locally by running consul, registering a proxy with the flag configured and starting Envoy with consul connect command.

PR Checklist

• updated test coverage
• external facing docs updated
• not a security concern

---

Overview of commits

• 8ff4b6f - a5498b9 - f0f7a3b - 28442ae - 4eb82e1 - 59d4378

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR

[freddygv]

Closing in favor of #16611, since this auto-backport is incomplete