core: fix MetricRecorderImpl#metricSinks ConcurrentModificationException

[JoeCqupt]

java.util.ConcurrentModificationException
    at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1013)
    at java.base/java.util.ArrayList$Itr.next(ArrayList.java:967)
    at io.grpc.internal.MetricRecorderImpl.addLongCounter(MetricRecorderImpl.java:95)
    at io.grpc.internal.SubchannelMetrics.recordConnectionAttemptSucceeded(SubchannelMetrics.java:82)
    at io.grpc.internal.InternalSubchannel$TransportListener$1.run(InternalSubchannel.java:605)
    at io.grpc.SynchronizationContext.drain(SynchronizationContext.java:96)
    at io.grpc.SynchronizationContext.execute(SynchronizationContext.java:128)
    at io.grpc.internal.InternalSubchannel$TransportListener.transportReady(InternalSubchannel.java:591)
    at io.grpc.netty.shaded.io.grpc.netty.ClientTransportLifecycleManager.notifyReady(ClientTransportLifecycleManager.java:51)

Fix #12727

[JoeCqupt]

@ejona86 PTAL