Remove namespace using in ETW exporter which affects global namespace

[ThomsonTan]

Changes

There is a using of namespace opentelemetry::exporter::etw at the global scope in etw_provider.h. It causes naming clash for any one who includes etw_provider.h which define the class name like Logger class.

This PR defines an alias on the namespace opentelemetry::exporter::etw instead of using it in global scope.

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

[lalitb]

Looks Good. Thanks for the fix.