Allow for ways to control logging/not-logging values on key by key basis

[ktoso]

Additional capability similar to privacy in os_log to be added to baggage keys.

A key has to define if it wants to be included in forEach and if it wants to be logged.

[...] mark a key as containing sensitive information . for example, you may have a key that carries sensitive user information and that you never want the collectors to log

via @tomerd