Proxy application metrics through envoy

[lahabana]

Description

Currently the application metrics are exposed as a regular service.
This is tricky if you want prometheus metrics scrape to not use mTLS.
See doc: https://kuma.io/docs/1.4.0/policies/traffic-metrics/#expose-metrics-from-applications

What could be done is for envoy to proxy metrics from the application as a service similar to the prometheus one.
This way we could easily use kuma-sd to also expose application metrics and not risk customers to accidentally expose more than necessary.

Once this is done it will probably make sense to disable mTLS on the listener exposing metrics by default (most of the time prometheus scrape is in plaintext).

It's currently unclear how this should be setup and requires some designing.

[lahabana]

This is tricky to setup on a per service base as currently TrafficMetrics are only part of the Mesh configuration.

[github-actions]

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[github-actions]

This issue was inactive for 30 days it will be reviewed in the next triage meeting and might be closed.
If you think this issue is still relevant please comment on it promptly or attend the next triage meeting.

[github-actions]

This issue was inactive for 30 days it will be reviewed in the next triage meeting and might be closed.
If you think this issue is still relevant please comment on it promptly or attend the next triage meeting.