fix(metrics): size exponential histogram exemplar reservoirs

[LouisDeconinck]

Fixes #2366.

The default exemplar reservoir for exponential histograms used the reservoir's CPU-count default instead of the specification's min(20, max_size) sizing rule.

This creates the default reservoir after validating max_size, with an explicit size capped at 20. Custom reservoirs are unchanged. The regression test covers smaller-than-20, equal-to-20, and larger-than-20 histograms.

Validation:

• git diff --check
• Focused Ruby tests not run: Ruby/Bundler is not installed in the execution environment.

AI assistance is disclosed in the commit trailer: Assisted-by: Codex.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: LouisDeconinck / name: Louis Deconinck (bee7677)

[Copilot]

🟢 Approval recommended

The change directly implements the spec-required default sizing and includes a focused regression test covering key boundary cases.

Pull request overview

This PR fixes the default exemplar reservoir sizing for exponential histograms in metrics_sdk to comply with the OpenTelemetry spec requirement of min(20, max_buckets) rather than using the reservoir’s CPU-count default.

Changes:

• Create the default SimpleFixedSizeExemplarReservoir only after validating max_size, explicitly setting max_size: [20, @size].min.
• Remove the shared DEFAULT_RESERVOIR constant so the default reservoir sizing can be derived from the histogram’s validated bucket limit.
• Add a regression test covering max_size below 20, equal to 20, and above 20.

File summaries

File	Description
metrics_sdk/lib/opentelemetry/sdk/metrics/aggregation/exponential_bucket_histogram.rb	Constructs the default exemplar reservoir with an explicit size capped at 20 based on validated histogram max_size.
metrics_sdk/test/opentelemetry/sdk/metrics/aggregation/exponential_bucket_histogram_test.rb	Adds a regression test asserting the default exemplar reservoir size follows the min(20, max_size) rule.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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[kaylareopelle]

Hi @LouisDeconinck, thank you for your contribution! Could you sign the CLA?

[simi]

Why to move out of private contstant?

[LouisDeconinck]

The default reservoir now needs a per-instance size - SimpleFixedSizeExemplarReservoir.new(max_size: [20, @SiZe].min) - which depends on the histogram's max_size , so it can't be a shared class constant. This also avoids all default-constructed histograms sharing a single mutable reservoir instance.

[simi]

Focused Ruby tests not run: Ruby/Bundler is not installed in the execution environment.

It would be good to at least somehow verify locally code works before submitting PR.

[LouisDeconinck]

@kaylareopelle CLA was signed.

[LouisDeconinck]

Verified locally in Docker (ruby:3.4): bundle exec rake test in metrics_sdk — 336 runs, 17,223 assertions, 0 failures — and bundle exec rake rubocop — 98 files, no offenses. The run includes the new sizing test ([4, 20, 32] -> [4, 20, 20] max_size cases).