Provide default implementation for MetricExporter trait methods

[cijothomas]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.8%. Comparing base (d52fec7) to head (31a6bf6).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/exporter.rs	66.6%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2479   +/-   ##
=====================================
  Coverage   77.8%   77.8%           
=====================================
  Files        123     124    +1     
  Lines      22864   22846   -18     
=====================================
- Hits       17800   17794    -6     
+ Misses      5064    5052   -12     

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[TommyCpp]

I think we should require exporter author to think through the expected behavior rather than have a blank implementation. The benifit I see is we reduce some duplication in our code base.

Also adding a default implemtation is not breaking change but removing the default implementation is. Thus I prefer to not add default implemtation. Overall I don't think the benifit is worth the risk