Prevent PROXY protocol clients from accessing metrics endpoint.

[sysvinit]

Due to an access control quirk, clients connecting to nginz over a listener which has proxy protocol enabled may be able to bypass the IP address access control list and access the /vts metrics endpoint, which is otherwised considered internal. This change prevents clients connected via proxy protocol from accessing the metrics endpoint.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If this PR changes development workflow or dependencies, they have been A) automated and B) documented under docs/developer/. All efforts have been taken to minimize development setup breakage or slowdown for co-workers.
• If HTTP endpoint paths have been added or renamed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • If new config options introduced: added usage description under docs/reference/config-options.md
  • If new config options introduced: recommended measures to be taken by on-premise instance operators.
  • If a cassandra schema migration is backwards incompatible (see also these docs), measures to be taken by on-premise instance operators are explained.
  • If a data migration (not schema migration) introduced: measures to be taken by on-premise instance operators.
  • If public end-points have been changed or added: does nginz need un upgrade?
  • If internal end-points have been added or changed: which services have to be deployed in a specific order?

[jschaul]

This fails, that variable does not exist:

│ 2022/04/26 16:00:22 [emerg] 8#0: unknown "proxy_protocol_address" variable                                                                                                                 │
│ nginx: [emerg] unknown "proxy_protocol_address" variable   

CI was also red here, PR should not get merged unless overall-ci is green.