fix(sdk-metrics) Don't Export from PeriodicExportingMetricReader with No Metrics

[JacksonWeber]

Which problem is this PR solving?

We should not attempt to export without metrics to export from the PeriodiicExportingMetricReader.

Fixes #5234

Short description of the changes

Adds a check to ensure that scope metrics are populated and updates related tests.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• should not export without populated scope metrics

Checklist:

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.62%. Comparing base (90afa28) to head (cd6c29f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5288   +/-   ##
=======================================
  Coverage   94.62%   94.62%           
=======================================
  Files         323      323           
  Lines        8068     8069    +1     
  Branches     1637     1638    +1     
=======================================
+ Hits         7634     7635    +1     
  Misses        434      434           

Files with missing lines	Coverage Δ
...etrics/src/export/PeriodicExportingMetricReader.ts	98.24% <100.00%> (+0.03%)	⬆️

[maryliag]

you have a lint error, otherwise LGTM

[hectorhdzg]

LGTM