Backport of [CC-4361] Add HCPMetricsBindSocketDir to Envoy bootstrap config into release/1.0.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #90 to be assessed for backporting due to the inclusion of the label backport/1.0.

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

---

This PR is very similar to hashicorp/consul#16511 in consul

Background

The HCP Cloud team is working on enabling observability metrics in HCP.
In particular, we want to forward Envoy metrics.

This PR aims to allow Envoy bootstrap configuration of 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.

Code changes

• internal/bootstrap/bootstrap_config.go : Add a HCPMetricsBindSocketDir string type in the BootstrapConfig struct and a function appendHCPMetricsConfig to generate the socket name, configure the stats sink and cluster for metrics forwarding.
• internal/bootstrap/bootstrap_config_test.go: add relevant tests to the above
• pkg/consuldp/bootstrap_test.go: Add a test to generate a golden test file
• internal/bootstrap/bootstrap_tpl.go : small change to move brackets [] into the template instead, which facilitates config generation

Next Steps

There will be a follow-up PR to generate a listener and cluster dynamically that will be listening on this 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

---

Overview of commits

• c6f02df - 8f2ab52 - 445aaf7 - f2bd67c