The Collector.Collect will be the field ns'Collect's callback, which be
invoked periodically with internal lock. And Collector.Add also runs
with ns.Lock in Collector.Lock, which is easy to cause deadlock.
Goroutine X:
ns.Collect
ns.Lock
Collector.Collect
Collector.RLock
Goroutine Y:
Collector.Add
Collector.Lock
ns.Lock
We should use ns.Lock without Collector.Lock in Add.
Fix: #6772
Signed-off-by: Wei Fu <fuweid89@gmail.com>
