Update grpc example to allow long runs

[psx95]

Updates the sample to add a --keepRunning flag that runs the sample program for 2 hours, generating a test span every 10 seconds. A successful run of this sample would verify that the per rpc credentials refresh is working as expected with the otlpgrpctraceexporter.

Testing

• go run example.go
• go run example.go --keepRunning

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.02%. Comparing base (4caace7) to head (c12f361).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #835      +/-   ##
==========================================
+ Coverage   61.03%   62.02%   +0.98%     
==========================================
  Files          56       56              
  Lines        5903     4850    -1053     
==========================================
- Hits         3603     3008     -595     
+ Misses       2143     1685     -458     
  Partials      157      157              

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