Expose monitoring package

[DomBlack]

What
Currently there is a monitoring package which allows the creation of encrypting and decryption loggers through the use of internalregistry.GetMonitoringClient(), Currently there is no way from outside of the tink-go library itself to set the monitoring client that is used by wrappedAead (or the monitoring loggers).

Why
I'd like to implement counters which monitor how many bytes are encrypted with our AEAD keys so we can implement key rotation automatically after a period of time or after a threshold of bytes has been written.

How
I'd like to propose that either we're given an alias to internalregistry.RegisterMonitoringClient or we're given a way via options to inject loggers at the point we call aead.New(...).

I understand the monitoring library is not production ready yet, however I think this would be a really useful addition to allow under a warning that you will need to maybe update your code in the future when the monitoring library changes.