Only set metrics in the current step (#289)

This changes the way we export gcloud metrics from using
`exportVariable` to setting `process.env` directly. The current approach
exports the environment variable for all current **and future** steps.
That means if a workflow uses deploy-appengine and then manually makes a
few gcloud calls in subsequent steps, all of those calls would be
attributed to deploy-appengine. It makes sense to do this for something
like setup-gcloud, where the attribution _should_ go to the installation
step. However, in this case, we are unintentionally attributing future
gcloud commands to deploy-appengine.

The new implementation only sets the values for the current process
environment. Even though that _shouldn't_ carry to future steps, it also
restores the previous environment values before exiting. Additionally,
it sets the "version" metric as well, so we can start tracking that.

Author: sethvargo