fix(InMemoryMetricExporter) clear metrics after shutdown

[paper2]

Which problem is this PR solving?

Fixes #5131

Short description of the changes

• make InMemoryMetricExporter reset storing metrics after shutdown

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

The InMemoryExporter does not directly affect the functionality of this library, but I consider it a minor specification change for the library. In fact, I encountered this issue while using InMemoryExporter in another project. If there are tests that retrieve metrics after shutdown, they will fail.

How Has This Been Tested?

• should get no metrics after shutdown

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.58%. Comparing base (fd7f2d9) to head (2bf3748).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5214      +/-   ##
==========================================
- Coverage   94.59%   94.58%   -0.02%     
==========================================
  Files         314      314              
  Lines        7993     7994       +1     
  Branches     1611     1611              
==========================================
  Hits         7561     7561              
- Misses        432      433       +1     

Files with missing lines	Coverage Δ
...s/sdk-metrics/src/export/InMemoryMetricExporter.ts	91.66% <100.00%> (+0.36%)	⬆️

... and 1 file with indirect coverage changes