Return true if this scoped guard will actively record metrics, and false otherwise. If the returned value is false the destructor will not record a value to the metric. A scoped guard will be inactive if either (1) it was not initialized with a valid metric, (2) the metric it was initialized with was not active at the time of construction, or (3) the metric supplied at construction is currently inactive, meaning the category of metrics this metric belongs to has been disabled since this object's construction (see the MetricsManager method setCategoryEnabled).