Add backend call counters
The idea is to add simple counters (vs full stats) for backend API method calls. For instance in #1418 (closed) it would be very fine to confirm that a lease is not updated twice. I know that profiling can get the same information but this is far more immediate for a low cost (i.e. I am sure the cost will be more than recovered by optimizations it is expected to allow).
Note we do not need large counters: it does not matter if a counter wraps as soon as it takes a long time to happen...
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