feat: exponential histogram - part 2 - the accumulation and aggregator

[mwear]

Which problem is this PR solving?

This PR is part 2 in a series of PRs to add exponential histogram support. See this comparison for the changes introduced in this PR (omitting the mapping functions from part 1).

Related: #3324

Short description of the changes

This PR includes ExponentialHistogramAccumulation and ExponentialHistogramAggregator along with extensive unit testing. This is the bulk of the work and uses the mapping functions introduced in part 1.

This code is heavily based on the Golang reference implementation. For other details see:

• the metrics data model spec
• the metrics sdk spec
• the metrics protobufs

For the other PRs in this series see:

• Part 1 - The mapping functions
• Part 2 - The accumulation and aggregator
• Part 3 - Wiring up export

You can see all 3 PRs combined in the original draft PR

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Extensive unit testing
• End to end test with the 3 PRs from this series adding ExponentialHistogram support

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

Merging #3505 (9434b41) into main (56e6b1b) will increase coverage by 0.16%.
The diff coverage is 97.53%.

❗ Current head 9434b41 differs from pull request most recent head 51a1b23. Consider uploading reports for the commit 51a1b23 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3505      +/-   ##
==========================================
+ Coverage   93.56%   93.72%   +0.16%     
==========================================
  Files         275      277       +2     
  Lines        8126     8448     +322     
  Branches     1691     1754      +63     
==========================================
+ Hits         7603     7918     +315     
- Misses        523      530       +7     

Impacted Files	Coverage Δ
.../packages/otlp-transformer/src/metrics/internal.ts	97.72% <ø> (ø)
.../aggregator/exponential-histogram/mapping/types.ts	100.00% <ø> (ø)
packages/sdk-metrics/src/export/MetricData.ts	100.00% <ø> (ø)
...sdk-metrics/src/aggregator/ExponentialHistogram.ts	97.15% <97.15%> (ø)
...cs/src/aggregator/exponential-histogram/Buckets.ts	98.01% <98.01%> (ø)
...r/exponential-histogram/mapping/ExponentMapping.ts	100.00% <100.00%> (ø)
.../exponential-histogram/mapping/LogarithmMapping.ts	100.00% <100.00%> (ø)
...trics/src/aggregator/exponential-histogram/util.ts	94.11% <100.00%> (+6.61%)	⬆️
packages/sdk-metrics/src/aggregator/types.ts	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[dyladan]

Sorry my review took so long

[dyladan]

@open-telemetry/javascript-maintainers would like at least one more set of experienced eyes on this. @legendecas if you have time to look I would really appreciate it since you're the most familiar with the metrics SDK. @pichlermarc you've also done a lot of metrics work so your insight here would also be great.

[pichlermarc]

Looks good. 🚀

I'm still having a hard time understanding everything, but I feel like I understand it a bit better now, thanks for adding all the great comments explaining what's happening. 🙂

No major comments, just a few nits. 🙂

[pichlermarc]

Was looking to merge this and did another quick review on it: I realized that we're exporting ExponentialHistogram already which may want to hold back until we get #3654 out the door.

However, with the proposed changes below we could merge it now and we won't have to wait for the 1.10.0 release. 🙂

[pichlermarc]

I think we may be able to reduce this to

Suggested change

	dataPoint:
	| DataPoint<number>
	| DataPoint<Histogram>
	| DataPoint<ExponentialHistogram>,
	dataPoint: DataPoint<number>,

if we change

• toSingularDataPoints(metricData: MetricData) -> toSingularDataPoints(metricData: SumMetricData | GaugeMetricData)
• toHistogramDataPoints(metricData: MetricData) -> toSingularDataPoints(metricData: HistogramMetricData)

This way, we don't have to export ExponentialHistogram from @opentelemetry/sdk-metrics and we can merge this PR before following through with #3654, as it would not expose any new types. 🙂

[mwear]

Sorry I was out last week. I wanted to check in to see if this is still needed since #3654 was merged and released?

[pichlermarc]

No worries 🙂
It's not blocking for me - since there's no release imminent I think it would be safe to merge as-is.