Flush pending telemetry when ForceFlush or Shutdown are called on a PeriodicReader

[MrAlias]

Resolve #3209

• Removes the output checking of the stdoutmetric exporter example. This output now includes an empty export from the unused sdk. It should be added back correctly in Add JSON encoder to stdoutmetric exporter that ignores timestamps #3223

[codecov]

Codecov Report

Merging #3220 (3f6932e) into main (8c6e6c4) will increase coverage by 0.0%.
The diff coverage is 91.4%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3220   +/-   ##
=====================================
  Coverage   77.3%   77.3%           
=====================================
  Files        159     159           
  Lines      11134   11167   +33     
=====================================
+ Hits        8610    8639   +29     
- Misses      2327    2331    +4     
  Partials     197     197           

Impacted Files	Coverage Δ
sdk/metric/periodic_reader.go	89.8% <91.4%> (-0.7%)	⬇️
exporters/jaeger/jaeger.go	90.3% <0.0%> (-0.9%)	⬇️
sdk/trace/batch_span_processor.go	81.9% <0.0%> (+0.8%)	⬆️