Unconditionally add Access-Control-Expose-Headers HTTP header

[NickCellino]

Description

This adds the following header to all Consul HTTP responses:

Access-Control-Expose-Headers: x-consul-default-acl-policy

The reasoning for this is in this RFC, but in summary, we need this header in order to support the HCP UI. Previously, we added this at startup conditionally if the cluster was being linked to HCP. With the changes in the RFC, we are allowing Consul to be linked to HCP at any time and adding this header unconditionally at startup is the simplest solution as it doesn't require us to reload configuration at runtime.

Testing & Reproduction steps

Manual test

Before

➜ http localhost:8500/v1/catalog/services
HTTP/1.1 200 OK
Content-Length: 2
Content-Type: application/json
Date: Tue, 16 Jan 2024 17:08:27 GMT
Vary: Accept-Encoding
X-Consul-Default-Acl-Policy: deny
X-Consul-Effective-Consistency: leader
X-Consul-Index: 19
X-Consul-Knownleader: true
X-Consul-Lastcontact: 0
X-Consul-Query-Backend: blocking-query

{}

After

➜  http localhost:8500/v1/catalog/services
HTTP/1.1 200 OK
Access-Control-Expose-Headers: x-consul-default-acl-policy
Content-Length: 2
Content-Type: application/json
Date: Tue, 16 Jan 2024 18:29:13 GMT
Vary: Accept-Encoding
X-Consul-Default-Acl-Policy: deny
X-Consul-Effective-Consistency: leader
X-Consul-Index: 18
X-Consul-Knownleader: true
X-Consul-Lastcontact: 0
X-Consul-Query-Backend: blocking-query

{}

Links

Jira: https://hashicorp.atlassian.net/browse/CC-7045
RFC: https://docs.google.com/document/d/185Kj_pDcuOmmVOzIPZYsCLIIOFB_26bg9kWD7i8qgkQ/edit#heading=h.5gihsi8y54bu

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

[mkam]

Looks good to me!