[EXPORTER] Fix forward protocol encoding for ETW exporter

[ThomsonTan]

The ETW exporter has code path for MsgPack serialization with macro HAVE_MSGPACK, but it stores all payload to an JSON object then serializes it to MsgPack as below which is incorrect.

opentelemetry-cpp/exporters/etw/include/opentelemetry/exporters/etw/etw_provider.h

Line 370 in c4f39f2

std::vector<uint8_t> v = nlohmann::json::to_msgpack(jObj);

The payload should follow fluent protocol as it is currently done in the fluent exporter in the contrib repo as below.

https://github.com/open-telemetry/opentelemetry-cpp-contrib/blob/main/exporters/fluentd/src/log/fluentd_exporter.cc#L95

Changes

Please provide a brief description of the changes here.

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (423ecac) 87.09% compared to head (bde8fb2) 87.09%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2473   +/-   ##
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  Coverage   87.09%   87.09%           
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  Files         200      200           
  Lines        6103     6103           
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  Hits         5315     5315           
  Misses        788      788           

[lalitb]

@ThomsonTan - Can you add some description about the changes, will help review this better :)

[lalitb]

LGTM. Thanks for the fix.

[marcalff]

Patch written by @ThomsonTan with approval from @lalitb (2 maintainers), small size, and localized to ETW.

This is sufficient to merge in my opinion, adding ok-to-merge label.