[EXPORTER] General cleanup for is_shutdown_ flags in exporters.

[marcalff]

This is a follow up on PR #2553, to simplify bool is_shutdown_ in general.

Changes

Please provide a brief description of the changes here.

• Replace every use of a spin lock + boolean by an std::atomic<bool>
• Replace a spin lock by a mutex, when serializing to a file in the ostream metric exporter
• Implemented the missing call to sout_.flush() in OStreamMetricExporter::ForceFlush()

For significant contributions please make sure you have completed the following items:

• [ ] CHANGELOG.md updated for non-trivial changes
• [ ] Unit tests have been added
• [ ] Changes in public API reviewed

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 87.54%. Comparing base (497eaf4) to head (60ce176). Report is 60 commits behind head on main.

Additional details and impacted files

[](https://app.codecov.io/gh/open-telemetry/opentelemetry-cpp/pull/2663?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) ```diff @@ Coverage Diff @@ ## main #2663 +/- ## ========================================== + Coverage 87.12% 87.54% +0.42% ========================================== Files 200 188 -12 Lines 6109 5840 -269 ========================================== - Hits 5322 5112 -210 + Misses 787 728 -59 ``` | [Files](https://app.codecov.io/gh/open-telemetry/opentelemetry-cpp/pull/2663?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) | Coverage Δ | | |---|---|---| | [...lemetry/exporters/memory/in\_memory\_span\_exporter.h](https://app.codecov.io/gh/open-telemetry/opentelemetry-cpp/pull/2663?src=pr&el=tree&filepath=exporters%2Fmemory%2Finclude%2Fopentelemetry%2Fexporters%2Fmemory%2Fin_memory_span_exporter.h&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwb3J0ZXJzL21lbW9yeS9pbmNsdWRlL29wZW50ZWxlbWV0cnkvZXhwb3J0ZXJzL21lbW9yeS9pbl9tZW1vcnlfc3Bhbl9leHBvcnRlci5o) | `89.48% <100.00%> (-1.43%)` | :arrow_down: | | [...de/opentelemetry/exporters/ostream/span\_exporter.h](https://app.codecov.io/gh/open-telemetry/opentelemetry-cpp/pull/2663?src=pr&el=tree&filepath=exporters%2Fostream%2Finclude%2Fopentelemetry%2Fexporters%2Fostream%2Fspan_exporter.h&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwb3J0ZXJzL29zdHJlYW0vaW5jbHVkZS9vcGVudGVsZW1ldHJ5L2V4cG9ydGVycy9vc3RyZWFtL3NwYW5fZXhwb3J0ZXIuaA==) | `100.00% <ø> (ø)` | | | [exporters/ostream/src/log\_record\_exporter.cc](https://app.codecov.io/gh/open-telemetry/opentelemetry-cpp/pull/2663?src=pr&el=tree&filepath=exporters%2Fostream%2Fsrc%2Flog_record_exporter.cc&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwb3J0ZXJzL29zdHJlYW0vc3JjL2xvZ19yZWNvcmRfZXhwb3J0ZXIuY2M=) | `97.06% <ø> (-0.08%)` | :arrow_down: | | [exporters/ostream/src/span\_exporter.cc](https://app.codecov.io/gh/open-telemetry/opentelemetry-cpp/pull/2663?src=pr&el=tree&filepath=exporters%2Fostream%2Fsrc%2Fspan_exporter.cc&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwb3J0ZXJzL29zdHJlYW0vc3JjL3NwYW5fZXhwb3J0ZXIuY2M=) | `87.88% <ø> (-0.24%)` | :arrow_down: | | [exporters/ostream/src/metric\_exporter.cc](https://app.codecov.io/gh/open-telemetry/opentelemetry-cpp/pull/2663?src=pr&el=tree&filepath=exporters%2Fostream%2Fsrc%2Fmetric_exporter.cc&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwb3J0ZXJzL29zdHJlYW0vc3JjL21ldHJpY19leHBvcnRlci5jYw==) | `90.91% <33.34%> (-0.89%)` | :arrow_down: | ... and [55 files with indirect coverage changes](https://app.codecov.io/gh/open-telemetry/opentelemetry-cpp/pull/2663/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry)