Metric tests intermittent failures about helidon

[spericas]

The issue is that a Histogram relies on an ExponentiallyDecayingReservoir where weighted sample data is stored. The weights are computed based on the time an item is added to the histogram. If the test does not add all the items within a second, the weights will differ and the test fails as shown above. This could easily happen on a slow machine or if artificial sleeps are introduced in the test while data is being added (with those added, the test fails every single time).

A potential workaround is to pass the timestamp for registration to ensure all data is registered at the same time, or at least pretend that to be the case. This is possible if histograms provide access to their underlying delegates and if some private access is changed to package private for testing purposes. A PR is on the way.