otlp: wrap the Cause error when context is canceled

[bboreham]

This brings things into alignment with the wait function.

I don't believe this is a "notable" change, so doesn't need a CHANGELOG entry.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.1%. Comparing base (f809f7d) to head (71c5417).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7811   +/-   ##
=====================================
  Coverage   86.1%   86.1%           
=====================================
  Files        302     302           
  Lines      22046   22046           
=====================================
  Hits       18991   18991           
- Misses      2674    2675    +1     
+ Partials     381     380    -1     

Files with missing lines	Coverage Δ
...s/otlp/otlplog/otlploggrpc/internal/retry/retry.go	97.6% <100.0%> (ø)
...s/otlp/otlplog/otlploghttp/internal/retry/retry.go	97.6% <100.0%> (ø)
.../otlpmetric/otlpmetricgrpc/internal/retry/retry.go	97.6% <100.0%> (ø)
.../otlpmetric/otlpmetrichttp/internal/retry/retry.go	97.6% <100.0%> (ø)
...lp/otlptrace/otlptracegrpc/internal/retry/retry.go	97.6% <100.0%> (ø)
...lp/otlptrace/otlptracehttp/internal/retry/retry.go	97.6% <100.0%> (ø)

... and 5 files with indirect coverage changes

🚀 New features to boost your workflow:

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

[pellared]

Can we add a new test case in TestWait that would cover the change?