Support for ExponentialHistogram in OTLP Registry

[lenin-jaganathan]

Resolves #3861.

Adds support for https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/data-model.md#exponentialhistogram. Math used for index calculation is re-used from the OTEL specification which lays down the formula/techniques to be considered for index calculation also keeping performance in mind.

Did some benchmarking between the explicit bucket histograms used earlier and the exponential histogram and below are the results,

GC allocation,

Throughput in ns,

Known Issues,

• For delta flavor, the support for last-minute data is not yet added.
• While recording the exact powers of 2 on a positive scale, the exact computation is not used. It seems it will cause performance concerns with extra computation for every recording cycle.

[sonatype-lift]

🛠 Lift Auto-fix

Some of the Lift findings in this PR can be automatically fixed. You can download and apply these changes in your local project directory of your branch to review the suggestions before committing.¹

# Download the patch
curl https://lift.sonatype.com/api/patch/github.com/micrometer-metrics/micrometer/3959.diff -o lift-autofixes.diff

# Apply the patch with git
git apply lift-autofixes.diff

# Review the changes
git diff

Want it all in a single command? Open a terminal in your project's directory and copy and paste the following command:

curl https://lift.sonatype.com/api/patch/github.com/micrometer-metrics/micrometer/3959.diff | git apply

Once you're satisfied, commit and push your changes in your project.

Footnotes

1. You can preview the patch by opening the patch URL in the browser. ↩

[lenin-jaganathan]

@jonatan-ivanov / @shakuzen Is this something that can be looked for next milestone release?

[lenin-jaganathan]

@shakuzen / @jonatan-ivanov Circling back to see if this can be looked upon?