For testing purposes, we want to unregister a previously registered
rate limiter prometheus metric and stop the goroutine that updates
this metric.
Signed-off-by: Ferran Rodenas <rodenasf@vmware.com>
`prometheus.MustRegister` panics if the metric cannot be registered.
Instead, use `prometheus.Register` and return the error if the metric
cannot be registered.
Also, add tests for `RegisterMetricAndTrackRateLimiterUsage`.
Signed-off-by: Ferran Rodenas <frodenas@gmail.com>