Rescale samples on reads too

[ashrayjain]

Previously, we were only rescaling on updates. This meant that
histograms would latch on to last updated values and stay at those
values until the next update for a reader who reads the histogram
periodically instead of decaying with time.

Fixes #215

[buffer51]

@rcrowley @mihasya

[bmoylan]

Friendly ping @rcrowley @mihasya

[mihasya]

This looks like a very good change, but it makes me wonder if folks have come to rely on the old behavior, and if changing this is a "backwards incompatible" change 🤔 Also trying to think of a case where this might cause a performance issue.. In a set up where you have lots of rarely updated counters that are read regularly, you could suddenly find yourself recomputing a bunch of EWMAs for things that were previously "free." Hmmm.

I do see a bunch of votes for this, and have definitely been annoyed by it in the past.

[p-kozlowski]

I'm worried the fix is not complete - Snapshot() is a read, so it needs rescaling, too. Otherwise, the reads performed on a Snapshoted metric will not trigger the rescaling.

If it seems out of scope of this MR, I could create another one. @ashrayjain, @mihasya what do you folks think about it?

[schmurfy]

any news on this ?

[ashrayjain]

@p-kozlowski you are correct. I have updated this to also rescale on Snapshot()