Allow Environment configs to report the metrics they expose

[bpkroth]

To support a dashboard GUI interface that allows creating new experiment variations from existing config structures ( #732 ), it would be useful for Environment configs to be able to declare what metrics one can expect for them to output from the scripts they run.

That way, an dashboard could examine the configs before they run, to provide a drop down of potential values to include in the objective_target(s) configs for a given experiment.