[BUILD] DLL export interface for Metrics

[perhapsmaple]

Fixes #2313

Changes

Add Metrics classes to exported DLL interface.

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: perhapsmaple / name: Harish Shan (b94d6ba, d25ca58, a177289, 606aebb, 80aaa15, d7c51ef, 26b2207, b1c4448, 96b1361)
• ✅ login: marcalff / name: Marc Alff (799461f, 3399256)
• ✅ login: ThomsonTan / name: Tom Tan (397e565)
• ✅ login: esigo / name: Ehsan Saei (4a08f95)

[codecov]

Codecov Report

Merging #2344 (4a08f95) into main (f16deb0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2344   +/-   ##
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  Coverage   87.43%   87.43%           
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  Files         199      199           
  Lines        6030     6030           
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  Hits         5272     5272           
  Misses        758      758           

[owent]

This is not a problem of this PR, but I think we should use if(OPENTELEMETRY_BUILD_DLL) instead of if(DEFINED OPENTELEMETRY_BUILD_DLL). And use target_compile_definitions(opentelemetry_api INTERFACE OPENTELEMETRY_BUILD_IMPORT_DLL) instead of add_definitions(-DOPENTELEMETRY_BUILD_IMPORT_DLL), so we can exporting this definition to cmake package.

[lalitb]

Thanks for the contribution. The PR looks good. The current CI is only executing traces example as here -

opentelemetry-cpp/ci/do_ci.ps1

Line 77 in e918960

examples\simple\Debug\example_simple.exe

Should we also add the metrics example here, to validate that the example is using the DLL correctly.

[perhapsmaple]

Hi, I have added both the metrics and logs tests to the cmake.dll.test workflow. Let me know if any other changes are required. Thanks

[lalitb]

Thanks. Good to have @ThomsonTan approval before merging, as he wrote the initial DLL code.

[ThomsonTan]

Thanks for the contribution.

[meastp]

@perhapsmaple thanks a lot for working on this! we discovered this today. we have been trying to modify otel-cpp locally to add metrics support for windows dll - we are however having some build issues. Could you please have a look, perhaps you can help us solve it? :) #2202 (comment)