Migrate opencensus bridge to errors.Join

[dashpole]

Split from #4585

Instead of aggregating error strings, use errors.Join to combine them. Error wrapping works properly now, and tests can check for specific error types.

[codecov]

Codecov Report

Merging #4599 (3901413) into main (1074d64) will decrease coverage by 0.1%.
The diff coverage is 82.6%.

@@           Coverage Diff           @@
##            main   #4599     +/-   ##
=======================================
- Coverage   81.3%   81.3%   -0.1%     
=======================================
  Files        222     222             
  Lines      17692   17683      -9     
=======================================
- Hits       14401   14390     -11     
- Misses      2991    2993      +2     
  Partials     300     300             

Files	Coverage Δ
bridge/opencensus/internal/ocmetric/metric.go	92.8% <82.6%> (-0.6%)	⬇️

... and 1 file with indirect coverage changes