subset lb doesn't properly remove subsets when being destroyed

[rgs1]

We use hot restarts for our deploys. Everything works nicely with the subset lb, except for the fact the lb_subsets_active gauge is wrong after the old process exits. That is, the number of active subsets in the old process doesn't get decremented. From a quick look at the code:

https://github.com/envoyproxy/envoy/blob/master/source/common/upstream/subset_lb.cc#L57

there's nothing going on in SubsetLoadBalancer::~SubsetLoadBalancer(), where we could probably do the decrements.

Any concerns with doing the decrements from the destructor? Is there a better – or more standard perhaps — way for dealing with gauges when in the context of hot restarts?

cc: @zuercher @mattklein123