Need a way to remove the guage child without being updated for certain period

[gangxie112]

In some case, the subject to monitor gone, left the guage with no updating, and if the value of the guage is beyound the alterting threshold, the alert is keeping firing. To avoid such case, we have to remember all the labels and remove the ones not exsisting. This makes that it's hard to decouple the monitoring and logic. If the client could provide a method to acheive this, it's much more easier to handle monitor out of the business logic.