Add options to opencensus bridge, and install tracer instead of returning

[dashpole]

Part of #4562

Changes:

• Add options to trace and metric bridges
• Add trace option to specify TracerProvider, which defaults to the global TracerProvider.
• Add InstallTraceBridge(), which simplifies the user experience by setting the DefaultTracer in OpenCensus.
• Deprecate NewTracer

[codecov]

Codecov Report

Merging #4567 (a50c982) into main (0022098) will decrease coverage by 0.1%.
The diff coverage is 77.7%.

@@           Coverage Diff           @@
##            main   #4567     +/-   ##
=======================================
- Coverage   81.3%   81.3%   -0.1%     
=======================================
  Files        220     221      +1     
  Lines      17669   17686     +17     
=======================================
+ Hits       14380   14391     +11     
- Misses      2989    2995      +6     
  Partials     300     300             

Files	Coverage Δ
bridge/opencensus/config.go	100.0% <100.0%> (ø)
bridge/opencensus/metric.go	100.0% <100.0%> (ø)
bridge/opencensus/trace.go	0.0% <0.0%> (ø)

... and 1 file with indirect coverage changes