[BUILD] Add bazel missing BUILD file

[tdat00]

Fixes

• None

Changes

• etw_threads example doesn't have BUILD file, we cannot run the example using Bazel as described here https://github.com/open-telemetry/opentelemetry-cpp/blob/main/CONTRIBUTING.md#build-and-run-code-examples
• This change adds the BUILD file for that example.

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: tdat00 / name: Dat Le (8f2e1a57132a099f8be15eae876199c83ce8474b)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.65%. Comparing base (497eaf4) to head (8f2e1a5). Report is 90 commits behind head on main.

Additional details and impacted files

[](https://app.codecov.io/gh/open-telemetry/opentelemetry-cpp/pull/2720?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) ```diff @@ Coverage Diff @@ ## main #2720 +/- ## ========================================== + Coverage 87.12% 87.65% +0.53% ========================================== Files 200 190 -10 Lines 6109 5853 -256 ========================================== - Hits 5322 5130 -192 + Misses 787 723 -64 ``` [see 107 files with indirect coverage changes](https://app.codecov.io/gh/open-telemetry/opentelemetry-cpp/pull/2720/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry)