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/selfsigned.crt

@@ -0,0 +1,11 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----