Allow metric name to be set via environment variable

At the moment, the Metric Name (and description) are statically settable only.
It would be great to have the possibility to use ENV variable, eg. similar to what is done with MQTT IN and OUT nodes.
I understand it can't be totally dynamic (through msg.payload.something). However, as ENV variables are initialized at runtime it would be a good solution. IMHO.
Thanks for the node anyway :)
