Fix exponential histogram bucket snapshot

[alanwest]

As @utpilla points out (#4303 (comment)), I have lost all ability to reason over CircularBufferBuckets. With some enhancements to my 🧠, I believe this PR makes the world right again.

[codecov]

Codecov Report

Merging #4313 (e96326b) into main (811d0eb) will increase coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head e96326b differs from pull request most recent head 2d76301. Consider uploading reports for the commit 2d76301 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4313      +/-   ##
==========================================
+ Coverage   84.50%   84.57%   +0.07%     
==========================================
  Files         298      298              
  Lines       11888    11899      +11     
==========================================
+ Hits        10046    10064      +18     
+ Misses       1842     1835       -7     

Impacted Files	Coverage Δ
src/OpenTelemetry/Metrics/CircularBufferBuckets.cs	100.00% <100.00%> (ø)
...enTelemetry/Metrics/ExponentialHistogramBuckets.cs	100.00% <100.00%> (ø)

... and 8 files with indirect coverage changes

[utpilla]

I think the unit tests should also cover a scale-down scenario when the CircularBufferBuckets offset is updated.

[alanwest]

It's not very obvious, but the existing tests actually do cover this scenario.

Even this simple test that only records two values requires a scale down. When the first value is recorded (10) the initial scale of 20 is fine, but when the second value is recorded (5) a scale down is necessary to fit this range of values within the default 160 buckets.

opentelemetry-dotnet/test/OpenTelemetry.Tests/Metrics/MetricSnapshotTests.cs

Lines 238 to 241 in d924663

	// SECOND EXPORT
	expectedHistogram.Record(5);
	histogram.Record(5);
	meterProvider.ForceFlush();

[utpilla]

Got it. Thanks!

[alanwest]

I'm going to merge this PR, but I will be coming back to the tests. Clearly there's improvements to make as the bug this PR fixes is not really covered by any tests yet.