Could `Family` class support limited number of metrics?

[zhitaoli-6]

As can be seen from the declaration of the class Family, there is only one method named Remove(), which can remove the metric added before.

prometheus-cpp/core/include/prometheus/family.h

Line 62 in 6492e82

class PROMETHEUS_CPP_CORE_EXPORT Family : public Collectable {

We worry about that there will be too many metrics if some label is added with very many values out of our expectation. So we want to limit the number of metrics for one Family object. Maybe LRU mechanism can get involved in to replace the very stale metric.