fix(sdk-metrics): collect metrics when periodic exporting metric reader flushes

[legendecas]

Which problem is this PR solving?

Fixes #3278

Related spec issue: open-telemetry/opentelemetry-specification#2983

Short description of the changes

• Collect immediately when the PeriodicExportingMetricReader flushes.

Type of change

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

How Has This Been Tested?

• PeriodicExportingMetricReader.forceFlush collects and exports metrics.

Checklist:

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

[codecov]

Codecov Report

Merging #3517 (14fbc00) into main (49c75b8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3517   +/-   ##
=======================================
  Coverage   93.80%   93.80%           
=======================================
  Files         249      249           
  Lines        7637     7640    +3     
  Branches     1589     1589           
=======================================
+ Hits         7164     7167    +3     
  Misses        473      473           

Impacted Files	Coverage Δ
...etrics/src/export/PeriodicExportingMetricReader.ts	98.00% <100.00%> (+0.12%)	⬆️

[pichlermarc]

Looks good, thanks for adding this. 🙂
I remember we agreed to remove it from the original PR about a year ago as it was not part of the spec, glad to see that it's back. 🙂

[aabmass]

Does this also cause metrics to be collected on shutdown?

[legendecas]

@aabmass No. This PR addresses the change in open-telemetry/opentelemetry-specification#3084.

To include effects of force flush in sdk shutdown, I've submitted open-telemetry/opentelemetry-specification#3085 and I can submit another PR to update it in JS SDK.

[aabmass]

Sounds good to me, thanks for following up!