[PROMETHEUS] Upgrade otelcpp to 1.28, add prometheus file exporter by owent · Pull Request #646 · open-telemetry/opentelemetry-cpp-contrib

owent · 2026-08-05T07:18:59Z

Fixes some thread-safety problems.
Implement prometheus file exporter.
Upgrade otelcpp to 1.28.

Copilot

Pull request overview

This PR updates the Prometheus exporter module to OpenTelemetry C++ 1.28 and adds a new Prometheus file exporter implementation alongside the existing push exporter.

Changes:

Bump Prometheus module versions/references from 1.19.0 to 1.28.0 (CMake/Bazel/GitHub Actions/vcpkg).
Add a new Prometheus file exporter (implementation, options, factory) and wire it into CMake + Bazel builds.
Extend Prometheus push exporter options/translation path with populate_target_info and without_otel_scope flags.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
exporters/prometheus/vcpkg.json	Adds vcpkg manifest for the Prometheus contrib module (now needs metadata aligned with push + file exporters).
exporters/prometheus/tools.cmake	Introduces CMake helpers for import/export visibility; contains Windows attribute and SunPro detection issues.
exporters/prometheus/src/push_exporter.cc	Plumbs new translation options through push exporter collection/translation.
exporters/prometheus/src/push_exporter_factory.cc	Exports the factory `Create` symbol via the new API macro.
exporters/prometheus/src/file_exporter.cc	Adds the new rotating-file exporter implementation (includes time/path formatting, background flush thread).
exporters/prometheus/src/file_exporter_factory.cc	Adds factory entry point for constructing the new file exporter.
exporters/prometheus/MODULE.bazel	Updates Bazel module + dependency versions to 1.28.0.
exporters/prometheus/include/opentelemetry/exporters/prometheus/push_exporter.h	Minor formatting-only change.
exporters/prometheus/include/opentelemetry/exporters/prometheus/push_exporter_options.h	Adds new options + API visibility macro definition.
exporters/prometheus/include/opentelemetry/exporters/prometheus/push_exporter_factory.h	Exposes factory `Create` with API macro and includes options header.
exporters/prometheus/include/opentelemetry/exporters/prometheus/file_exporter.h	Adds public header for the new file exporter (needs API export annotation).
exporters/prometheus/include/opentelemetry/exporters/prometheus/file_exporter_options.h	Adds options + API visibility macro definition for file exporter.
exporters/prometheus/include/opentelemetry/exporters/prometheus/file_exporter_factory.h	Adds public factory header for the file exporter.
exporters/prometheus/CMakeLists.txt	Updates project version, adds new library target, and adds import/export macro wiring.
exporters/prometheus/BUILD	Adds Bazel `cc_library` target for the file exporter.
.github/workflows/prometheus.yml	Updates CI to test against opentelemetry-cpp v1.28.0.

Suppressed comments (3)

exporters/prometheus/tools.cmake:44

Same issue as the export path: __attribute__((__dllimport__)) is not the standard GCC/Clang attribute spelling. This likely results in no import decoration on Windows with MinGW/Clang.

  if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang|AppleClang|Intel|XL|XLClang")
    if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
      set(${OUTPUT_VARNAME}
          "__attribute__((__dllimport__))"
          PARENT_SCOPE)

exporters/prometheus/src/file_exporter.cc:693

~PrometheusFileBackend() joins the background flush thread but never sets is_shutdown to stop it. The thread can run for up to 1 minute before exiting (idle timeout), so destroying the exporter can block for a long time if Shutdown() wasn’t called.

  ~PrometheusFileBackend()
  {
    if (file_)
    {
      file_->background_thread_waker_cv.notify_all();
      std::unique_ptr<std::thread> background_flush_thread;

exporters/prometheus/src/file_exporter.cc:1089

CheckUpdate() updates rotate_index and calls ResetLogFile() without holding file_lock, even though ResetLogFile() explicitly assumes it is called under lock. This is a data race if AddMetricData() can be invoked concurrently, and can also conflict with OpenLogFile()/RotateLog() which use the same state under file_lock.

    // Reset rotate index when directory changes
    if (new_dir != old_dir)
    {
      file_->rotate_index = 0;
    }

    ResetLogFile();

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owent · 2026-08-05T15:18:36Z

+class PrometheusFileExporter : public ::opentelemetry::sdk::metrics::PushMetricExporter
+{


It's not public header

owent · 2026-08-09T13:30:52Z

Sorry for it's be a long time to update this package. Could you please review it again when you have time? @ThomsonTan @esigo @lalitb

proost

Maintainers looks very busy, so i'd like to help out.

Can you add test cases for file exporter?

proost · 2026-08-12T10:58:16Z

+      return 0;
+    }
+
+    file.seekg(std::ios::end);


file.seekg(0, std::ios::end)

Thanks, unit tests are added.

proost · 2026-08-12T11:05:35Z

+    std::string new_dir = FileSystemUtil::DirName(new_file_path);
+    std::string old_dir = FileSystemUtil::DirName(old_file_path);
+
+    // Reset rotate index when directory changes


Looks like we should hold lock in here to call ResetLogFile. Am i correct?

It's not required to be very precise in file size calculation. So I changed all the variables in ResetLogFile tobe atomic.And no lock is reauired neither.

proost · 2026-08-12T11:18:43Z

+    file_->flushed_metric_family_count.store(0);
+  }
+
+  ~PrometheusFileBackend()


Are there no need to set "is_shutdown" to true?

Thanks, is_shutdown is set now.

proost · 2026-08-19T12:28:29Z

+        std::lock_guard<std::mutex> lock_guard{file_->background_thread_lock};
+        if (!file_->background_flush_thread)
+        {
+          break;


non-blocking; no background flush thread and "is_shutdown" is true, can we return earlier right?

Sorry, I'm not sure I follow. When there's been no IO for a while, the background thread exits on its own, so it shouldn't matter whether it's explicitly shut down or not.

owent · 2026-09-15T03:08:50Z

@proost Could you please review this PR again when you have time? Thanks.

proost

Thanks!

proost · 2026-09-15T12:18:12Z

+  bool ForceFlush(std::chrono::microseconds timeout) noexcept
+  {
+    std::chrono::microseconds wait_interval = timeout / 256;
+    if (wait_interval <= std::chrono::microseconds{0})


non-blocking; Maybe guarding unrealistic too large value too?

Copilot AI lite review requested due to automatic review settings August 5, 2026 07:19

owent requested a review from a team as a code owner August 5, 2026 07:19

Copilot started reviewing on behalf of owent August 5, 2026 07:19 View session

Copilot AI reviewed Aug 5, 2026

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owent force-pushed the prometheus_exporters branch from 7b153bb to da9b374 Compare August 5, 2026 12:43

owent mentioned this pull request Aug 9, 2026

Update OtlpGrpcClientOptions to populate options for gRPC client sharing open-telemetry/opentelemetry-cpp#4249

Open

proost reviewed Aug 12, 2026

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owent force-pushed the prometheus_exporters branch from 436105b to ac28893 Compare August 17, 2026 06:21

proost reviewed Aug 19, 2026

View reviewed changes

owent added 10 commits August 20, 2026 14:17

[PROMETHEUS] Upgrade otelcpp to 1.28, add prometheus file exporter

2bb931e

Update bazel modules

996ebef

Fixes bazel and cmake support

586d5f8

Fixes prometheus linking

b615728

Fixes document

62b6e3f

Fixes compatibility

75c70e8

Fixes compatibility of localtime_s usage

2136e6a

Make all variables in ResetLogFile atomic

4d30b05

Add unit test for PrometheusFileExporter

ec4bec3

Merge concunrrency fixes from <open-telemetry/opentelemetry-cpp#4365>

4156383

owent force-pushed the prometheus_exporters branch from ba10b0e to 4156383 Compare August 20, 2026 06:18

owent requested a review from proost September 15, 2026 03:07

proost approved these changes Sep 15, 2026

View reviewed changes

		class PrometheusFileExporter : public ::opentelemetry::sdk::metrics::PushMetricExporter
		{

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owent commented Aug 9, 2026

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proost Aug 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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owent commented Sep 15, 2026

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owent commented Aug 5, 2026 •

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proost Aug 19, 2026 •

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