Propose to add GetMetricWithLabelvaluesIfExist fuction to XXXVec struct

[ethanvc]

Occasionally, developer will set label values that unable to converge, which make lots of metric series. In the worst case, it will oom the target process or consume lots of cpu/memory.

So I want to add a policy to protect my system, the step is:

1. If the total metric series less then xx count, just do as before(Inc or Add).
2. If the total metric series above xx count, add an extra check: if the metric already exist, do as before(because just +/-, no extra memory need), otherwise drop the monitor request and report a error monitor event.

To fulfill above protection policy, I need to know if a specified Counter already exist.

So, I hope maybe we can add function like GetMetricWithLabelvaluesIfExist(maybe not this name but have same meaning) to acquire this information: if i get the metric successfully, return the Counter, otherwise return error.