add metrics parameter to N1qlParams

There is a parameter for N1QL that allows for disabling the sending of metrics. It's covered in the N1QL Query API Request Parameters. It would be good to add this to the N1qlParams to be able to save transport/memory for situations where it's not needed.

I checked with Keshav Murthy and the param has been available since 4.0, but isn't currently in the docs on developer.couchbase.com. He'll handle documentation on it in the future.