ElasticsearchLogRecordableTests.BasicTests fails on Ubuntu 24.04 with CXX_STANDARD=20

[dbarker]

Describe your environment
main at 3ca3c76
Ubuntu 24.04 based devcontainer

Steps to reproduce

• Build with CXX_STANDARD=20
• Run ./ci/do_ci.sh cmake.maintainer.async.test

What is the expected behavior?
test passes

What is the actual behavior?
ElasticsearchLogRecordableTests.BasicTests fails.

[----------] 1 test from ElasticsearchLogRecordableTests
[ RUN      ] ElasticsearchLogRecordableTests.BasicTests
/workspaces/opentelemetry-cpp/exporters/elasticsearch/test/es_log_record_exporter_test.cc:127: Failure
Expected equality of these values:
  actual
    Which is: {"@timestamp":"2024-11-20T00:52:24.999647774+00:00","boolean":false,"double":2.0,"ecs":{"version":"8.11.0"},"int":1,"log":{"level":"FATAL","logger":"scope_name"},"message":"Body of the log message","observedtimestamp":1732063944999647774,"stringlist":["string1","string2"]}
  expected
    Which is: {"@timestamp":"2024-11-20T00:52:24.999647Z","boolean":false,"double":2.0,"ecs":{"version":"8.11.0"},"int":1,"log":{"level":"FATAL","logger":"scope_name"},"message":"Body of the log message","observedtimestamp":1732063944999647774,"stringlist":["string1","string2"]}

[  FAILED  ] ElasticsearchLogRecordableTests.BasicTests (0 ms)
[----------] 1 test from ElasticsearchLogRecordableTests (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (0 ms total)
[  PASSED  ] 0 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] ElasticsearchLogRecordableTests.BasicTests

 1 FAILED TEST

[sjinks]

https://github.com/open-telemetry/opentelemetry-cpp/blob/main/exporters/elasticsearch/src/es_log_recordable.cc#L95

std::format() uses different precision

If the precision of the input cannot be exactly represented with seconds, then the format is a decimal floating-point number with a fixed format and a precision matching that of the precision of the input (or to a microseconds precision if the conversion to floating-point decimal seconds cannot be made within 18 fractional digits)

[dbarker]

https://github.com/open-telemetry/opentelemetry-cpp/blob/main/exporters/elasticsearch/src/es_log_recordable.cc#L95

std::format() uses different precision

If the precision of the input cannot be exactly represented with seconds, then the format is a decimal floating-point number with a fixed format and a precision matching that of the precision of the input (or to a microseconds precision if the conversion to floating-point decimal seconds cannot be made within 18 fractional digits)

Good catch!

[sjinks]

It makes sense to remove that conditional. As far as I know, there's no easy way to make std::format do proper rounding. We will either reinvent our own implementation of strftime() or have to use format() + substr(). But because format() is slower, the existing strftime() approach seems to be the best option.