feat(metrics): enable TLS/mTLS serving support (#1576)

* feat(config): add metrics TLS configuration

Signed-off-by: Xe Iaso <me@xeiaso.net>

* feat(metrics): add naive TLS serving for metrics

Signed-off-by: Xe Iaso <me@xeiaso.net>

* feat(metrics): import keypairreloader from a private project

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(metrics): properly surface errors with the metrics server

Signed-off-by: Xe Iaso <me@xeiaso.net>

* feat(config): add CA certificate config value

Signed-off-by: Xe Iaso <me@xeiaso.net>

* feat(metrics): enable mTLS support

Signed-off-by: Xe Iaso <me@xeiaso.net>

* doc(default-config): document how to set up TLS and mTLS

Signed-off-by: Xe Iaso <me@xeiaso.net>

* doc: document metrics TLS and mTLS

Signed-off-by: Xe Iaso <me@xeiaso.net>

* chore: spelling

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/config/testdata/tls/minica-key.pem

@@ -0,0 +1,6 @@
+-----BEGIN PRIVATE KEY-----
+MIG2AgEAMBAGByqGSM49AgEGBSuBBAAiBIGeMIGbAgEBBDDr9QQo7ZaTgUL6d73G
+2BG7+YRTFJHAZa0FogRglfc+jYttL1J4/xTig3RmHoqSgrehZANiAASDhijM9Xe0
+G9Vam6AJMeKC6aWDNSLwrxNVmPxemsY/yJ1urBgnxRd9GFH6YW1ki/B8rS+Xl1UX
+NnhBrukLaXvgAQQq782/5IUYGsvK5jw8+dSscYVMCQJwGfmQuaNeczQ=
+-----END PRIVATE KEY-----