otlp_grpc_exporter export after shutdown should fail

[esigo]

according to the exporter specifications, export after shutdown should fail

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1050 (3166535) into main (52f6198) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1050      +/-   ##
==========================================
- Coverage   94.85%   94.83%   -0.01%     
==========================================
  Files         151      151              
  Lines        5971     5971              
==========================================
- Hits         5663     5662       -1     
- Misses        308      309       +1     

Impacted Files	Coverage Δ
...include/opentelemetry/sdk/common/circular_buffer.h	97.92% <0.00%> (-2.08%)	⬇️

[lalitb]

Thanks for the fix.