Limit metrics to be collected

[dannyk81]

Hey! first of, thanks for this work 👍

We are now testing this exporter in our environment, one thing we noticed that it takes ~1m32.703s to scrape the vcenter metrics, probably as our environment is quite big.

I wonder if it's possible to limit which metrics should be collected? for example if we only want datastore metrics?

[pryorda]

This issue was moved to pryorda/vmware_exporter#5