Collect in PeriodicExportingMetricReader ForceFlush before exporting

[johanpel]

Is your feature request related to a problem?

Consider the scenario where some executable emitting metrics through the PeriodicExportingMetricReader is about to exit, but desires to collect and export the last value of an ObservableGauge one more time without waiting for the periodic collect and export.

Describe the solution you'd like

When using the PeriodicExportingMetricReader, it would be nice if ForceFlush() collects before exporting.

Describe alternatives you've considered
I have a metric that could be a Counter, but it gets updated very often and emits too many metric points. Therefore I really want to just collect and export it periodically using an ObservableGauge. I could work around it by using a Counter and periodically updating that myself, forcing one final update and flush upon exit. However, it seems the behavior I desire is common among other language SDKs as well, so I think it may be desired by others for ForceFlush to follow the behavior seen elsewhere.

Additional context

I hope I am not missing anything, but if I understand correctly, in the current implementation, an export is performed but no metrics are collected before exporting.
https://github.com/open-telemetry/opentelemetry-cpp/blob/main/sdk/src/metrics/export/periodic_exporting_metric_reader.cc#L85-L88

Other language SDKs appear to collect before exporting on ForceFlush:

Java: https://javadoc.io/static/io.opentelemetry/opentelemetry-sdk-metrics/1.21.0/io/opentelemetry/sdk/metrics/export/PeriodicMetricReader.html#forceFlush()
Python (see force_flush at the bottom, the superclass force_flush collects first): https://opentelemetry-python.readthedocs.io/en/latest/_modules/opentelemetry/sdk/metrics/_internal/export.html#PeriodicExportingMetricReader.force_flush

[lalitb]

I hope I am not missing anything, but if I understand correctly, in the current implementation, an export is performed but no metrics are collected before exporting.

Yes, this is correct. Thanks for raising the issue.

Related issues on specs:
open-telemetry/opentelemetry-specification#2924
open-telemetry/opentelemetry-specification#2983

[lalitb]

The MetricReader::ForceFlush() method needs more clarity from specification prospective. As of now, python and java does collection while JS and C++ doesn't.
Regarding the scenario described in this issue, it would make more sense for PeriodicExportingMetricReader::Shutdown() to do one last collection to capture all metrics before exiting ?

[johanpel]

The MetricReader::ForceFlush() method needs more clarity from specification prospective

Agreed, thanks for linking the related issues.

PeriodicExportingMetricReader::Shutdown() to do one last collection to capture all metrics before exiting

Yes, that would cover the scenario perfectly.

[sfc-gh-sili]

@lalitb @johanpel Hello! Is there any reason PeriodicExportingMetricReader don't collect another round of data on Shutdown anymore? ref:

opentelemetry-cpp/sdk/src/metrics/export/periodic_exporting_metric_reader.cc

Line 74 in 2320636

bool PeriodicExportingMetricReader::CollectAndExportOnce()