[BUILD] Fix missing exported definition for OTLP file exporter and forceflush#3319

marcalff merged 3 commits intoopen-telemetry:mainopen-telemetry/opentelemetry-cpp:mainfrom

ThomsonTan:fix_dll_otlp_fileThomsonTan/opentelemetry-cpp:fix_dll_otlp_fileCopy head branch name to clipboard