feat: add InMemoryMetricExporter

[weyert]

Which problem is this PR solving?

Introduces the InMemoryMetricExporter-class which collects metrics and stores it in memory

Fixes #3035 (issue)

Short description of the changes

Adds the InMemoryMetricExporter-class

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Unit tests

Checklist:

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: tapico-weyert (1f208a1)

[codecov]

Codecov Report

Merging #3039 (b065758) into main (1d0eaef) will increase coverage by 0.02%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##             main    #3039      +/-   ##
==========================================
+ Coverage   93.09%   93.11%   +0.02%     
==========================================
  Files         188      189       +1     
  Lines        6253     6276      +23     
  Branches     1318     1319       +1     
==========================================
+ Hits         5821     5844      +23     
  Misses        432      432              

Impacted Files	Coverage Δ
...-metrics-base/src/export/InMemoryMetricExporter.ts	91.30% <91.30%> (ø)
...s-base/src/export/PeriodicExportingMetricReader.ts	95.65% <0.00%> (+4.34%)	⬆️

[weyert]

@open-telemetry/javascript-approvers ready to review I think :)

[legendecas]

Sorry for the churn, but I find some disagreement in the review comments.

[jkwatson]

@open-telemetry/java-instrumentation-approvers ready to review I think :)

unclear why java folks should be involved here...

[dyladan]

@weyert hope you don't mind I went ahead and made the changes so we can get this merged. @legendecas please take another quick look so we can get this merged.