chore: make 'cargo test' in the opentelemetry-sdk dir work

[scottgerring]

There are some doctests in opentelemetry-sdk that prevent cargo test from running in the subdirectory. This is confusing for new-starters on the project and is easy enough to fix!

I am hesitant to add this to scripts/test.sh as we are already running so many variants but I welcome opinions here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.8%. Comparing base (df412fe) to head (d371084).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3258   +/-   ##
=====================================
  Coverage   80.8%   80.8%           
=====================================
  Files        129     129           
  Lines      23203   23203           
=====================================
+ Hits       18750   18751    +1     
+ Misses      4453    4452    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.