Docs: improve how metadata are enabled

[sebastian-alfers]

https://support.lightbend.com/agent/lightbend/lightbend/tickets/details/427921000059888054

The docs currently do not contain the fact that it is necessary to explicitly enable headers that are allowed to be forwarded.

[johanandren]

They do document it, but only for the spring/code first SDK, not for the protobuf ones: https://docs.kalix.io/java/actions.html#_forwarding_headers