`PeriodicExportingMetricReader::Shutdown` should include effects of `ForceFlush`

[danielgblanco]

Per open-telemetry/opentelemetry-specification#2983 and its fix, open-telemetry/opentelemetry-specification#5305, the metrics spec would require the periodic exporting MetricReader to collect and export whatever metrics were recorded since the last periodic tick before the underlying exporter is shut down.

I'm not experienced in opentelemetry-cpp or its codebase, but the following AI-assisted analysis seems to confirm PeriodicExportingMetricReader::Shutdown doesn't currently do this:

• MetricReader::Shutdown sets the shutdown flag before calling OnShutDown.
• OnShutDown just wakes the worker thread, joins it, and shuts the exporter down — no extra collect.
• The worker's own loop exits on that wakeup without re-entering its collect-and-export step (CollectAndExportOnce), since the shutdown flag is checked after waking up, not before.
• OnForceFlush's wait condition returns immediately once IsShutdown() is true, without the worker having collected anything — so calling ForceFlush() manually doesn't help either once Shutdown() has started (calling it before Shutdown() does work today, for what it's worth).

Metrics recorded since the last periodic tick appear to be silently dropped on shutdown. I'd like to know if it's possible for Shutdown() to perform one last collect-and-export cycle (the same effect as ForceFlush() or just calling ForceFlush()) before the exporter is shut down.

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[nikhilbhatia08]

Hey @dbarker I can work on this issue, please can you assign this issue to me, Thanks!

[dbarker]

Thanks for the report @danielgblanco. This does seem like a spec compliance gap and is needed. It may also be an issue for the span/log batch processors as well.

Hi @nikhilbhatia08, Thanks for taking this on! Please link the PR with this issue so others will see it is in progress.