[Exporter.Prometheus] Improve performance#7279
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martincostello merged 100 commits intoJun 5, 2026
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- Add support for configuring OpenTelemetry.Exporter.Prometheus.HttpListener with the `OTEL_EXPORTER_PROMETHEUS_HOST` and `OTEL_EXPORTER_PROMETHEUS_PORT` environment variables. - Remove field for UriPrefixes and use auto-property. - Remove `UriPrefixes` from the README. Fixes open-telemetry#4158. Fixes open-telemetry#7154.
Fix markdownlint warnings.
Add coverage for invalid environment variables.
Remove extra semicolons.
React to changes from open-telemetry#7175.
Remove duplicated constructor declaration.
- Add missing SHOULD requirement to specify the `escaping` value for 1.0.0 protocols. - Update `PrometheusSerializer` to be compliant with `escaping=underscores` when using OpenMetrics.
Fix missing prefixing for metrics that start with a digit.
- Use canonical representations for numbers for "le" label values of histograms and "quantile" label values of summary metrics for OpenMetrics. - Resolve TODO by moving check outside loop. See https://prometheus.io/docs/specs/om/open_metrics_spec/#considerations-canonical-numbers.
Add CHANGELOG entries.
Remove branches that could not be reached.
- Check destination size. - Update CHANGELOGs.
Fix incorrect serialized value for `PrometheusType.Untyped` when using OpenMetrics.
Omit histogram `_sum` and `_count` in OpenMetrics when negative bucket thresholds are present.
Update PR number.
Export `{name}_created` series for counters and histograms when start time is available when using OpenMetrics.
Add missing `TYPE` metadata.
- Remove non-spec `TYPE` for `_counter`. - Fix-up timestamp precision.
Fix-up duplicated definitions.
Fix-up merge.
Add missing suffix.
- Emit OpenMetrics scope metadata as a single `otel_scope` metric family with `otel_scope_info` samples instead of repeating metadata for every scope. - Include instrumentation scope metadata on samples using `otel_scope_*` labels, including scope version, schema URL, and prefixed scope attributes. - Drop conflicting scope attributes named `name`, `version`, and `schema_url` to avoid collisions with generated scope labels.
Remove Go theme for .NET.
Add CHANGELOG entries.
Address Copilot review feedback.
Add more test coverage for patch.
Add Prometheus text fallback `target_info` output as a gauge so resource metadata is still exposed as Info-typed metrics are unavailable for PrometheusText exposition format.
Add PR number.
Merge colliding sanitized label keys by concatenating values in lexicographic order of the original keys.
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Revert some changes from merge with main which aren't needed.
Restore optimisations that were lost in merge.
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Pull request overview
This PR targets performance improvements in the Prometheus HttpListener exporter serialization path by reducing per-metric string conversions, optimizing escaping and numeric formatting on newer TFMs, and reusing pre-serialized data (metric names and tag sets) across repeated writes (notably histogram bucket output).
Changes:
- Add faster escaping implementations (using
SearchValues<char>on .NET 8+) and write common label value types directly to UTF-8. - Cache pre-serialized metric name / metadata name / unit bytes on
PrometheusMetricand write them as UTF-8 bytes instead of char-by-char loops. - Pre-serialize histogram tag sets once per
MetricPointand reuse across bucket/sum/count output; broaden buffer-growth retry logic to includeArgumentExceptionin addition toIndexOutOfRangeException.
Reviewed changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.
| File | Description |
|---|---|
| src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializerExt.cs | Reuse pre-serialized tag bytes for histogram output and add helpers for writing serialized tag spans. |
| src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs | Optimize escaping and label value formatting; write cached metric/unit bytes directly to the output buffer. |
| src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusMetric.cs | Cache ASCII byte representations of metric-related names/units; adjust OpenMetrics naming/unit handling. |
| src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs | Treat ArgumentException as a buffer-growth signal during serialization retries (like IndexOutOfRangeException). |
Comments suppressed due to low confidence (3)
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs:268
- This
catch (Exception ex) when (ex is IndexOutOfRangeException or ArgumentException)introduces an unusedexvariable and is less explicit than catching the two expected exception types directly. CatchingIndexOutOfRangeExceptionandArgumentExceptionseparately keeps the intent clear and avoids the unused variable.
catch (Exception ex) when (ex is IndexOutOfRangeException or ArgumentException)
{
if (!IncreaseBufferSize(ref buffer))
{
throw;
}
}
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs:285
- This
catch (Exception ex) when (ex is IndexOutOfRangeException or ArgumentException)introduces an unusedexvariable and is less explicit than catching the two expected exception types directly. CatchingIndexOutOfRangeExceptionandArgumentExceptionseparately keeps the intent clear and avoids the unused variable.
catch (Exception ex) when (ex is IndexOutOfRangeException or ArgumentException)
{
if (!IncreaseBufferSize(ref buffer))
{
throw;
}
}
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs:337
- This
catch (Exception ex) when (ex is IndexOutOfRangeException or ArgumentException)introduces an unusedexvariable and is less explicit than catching the two expected exception types directly. CatchingIndexOutOfRangeExceptionandArgumentExceptionseparately keeps the intent clear and avoids the unused variable.
catch (Exception ex) when (ex is IndexOutOfRangeException or ArgumentException)
{
if (!IncreaseBufferSize(ref buffer))
{
throw;
}
}
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Revert change from merge that is now redundant (and wrong).
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Copilot summary of comparing the results from running
Largest improvements were in |
martincostello
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Jun 5, 2026
Shorten to just NET.
Kielek
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Jun 5, 2026
Kielek
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One NIT + some codex feedback
[P2] src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs:257: the new typed label-value formatter is only used on the fast path. When WriteTags falls back for sanitized key collisions, AddLabel still stores values through GetLabelValueString at line 956, which uses current-culture ToString() for everything except bool/double/float at line 570. That makes the same label value serialize differently depending on whether a collision occurs. For example, under fr-FR, a non-colliding decimal point tag now emits 1.23, but colliding tags that go through merge can emit 1,23;.... The fallback path should use the same invariant formatting logic as WriteLabelValue(object?).
- Add comment. - Fix slow-path to also use invariant formatting. - Simplify some tests.
Kielek
approved these changes
Jun 5, 2026
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Changes
Improve the performance of
PrometheusSerializerby:SearchValues<T>on .NET 8+ToString()See #7279 (comment) for results.
Supersedes #7170.
Merge requirement checklist
AppropriateCHANGELOG.mdfiles updated for non-trivial changesChanges in public API reviewed (if applicable)