Major Description
At sensor.java line:254, we call `this.metrics.values()`. metrics is not a thread safe map and that is why we acquire a lock whenever we want to add/remove entries from it. For example, see add(), hasMetrics() method.
However, we don't acquire a lock when calling Sensor#checkQuotas(timeMs).
This could lead to a situation where this metrics map may be left in an inconsistent state (since it is not thread safe for concurrent read/write access).
The objective of this task is to validate what I said above is correct and if yes, then fix the situation by enclosing this read in a lock. As a stretch task, we should consider if we can replace the metrics data structure which allows concurrent reads but exclusive writes.