Add `account_name` label to the metrics

[ercpereda]

I would like to monitor the rate limit for different github accounts in the same prometheus.
I can't find a way to do it; even if I deploy multiple instance of the exporter, the metrics will collide.
My propouse is to add the label account_name to the metrics, then metrics from differents accounts can be differentiated.
So, we'll end up with something like
github_rate_limit{account_name=ercpereda}