[SDK] PeriodicExportingMetricReader: future is never set, blocks until timeout

[owent]

Fixes #3026

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

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.62%. Comparing base (497eaf4) to head (35ce020).
Report is 119 commits behind head on main.

Files	Patch %	Lines
...metrics/export/periodic_exporting_metric_reader.cc	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3030      +/-   ##
==========================================
+ Coverage   87.12%   87.62%   +0.51%     
==========================================
  Files         200      190      -10     
  Lines        6109     5872     -237     
==========================================
- Hits         5322     5145     -177     
+ Misses        787      727      -60     

Files	Coverage Δ
...metrics/export/periodic_exporting_metric_reader.cc	79.81% <80.00%> (+0.46%)	⬆️

... and 122 files with indirect coverage changes

[marcalff]

LGTM, thanks for the fix.

[esigo]

LGTM
thanks for the PR :)