Change InMemoryExporter to clear collection before exporting Metrics

[TimothyMothra]

Fixes #2361

Changes

• New class InMemoryMetricExporter which inherits InmemoryExporter.
  This new class will clear the exported collection before exporting Metrics.
  This mitigates the issue with growing the collection with copies of the same Metric.
• Minor changes InMemoryExporter to allow code reuse.
• update doc to explain the contract of MetricExporter
• update existing unit tests, no longer need to manually clear.
• add new tests, exploring the behavior of working with Metrics.

TODO

• Need help with statement in the Readme

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue InMemoryExporter for Metric is buggy #2361
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #2937 (83af046) into main (bdcf942) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 83af046 differs from pull request most recent head cfb8773. Consider uploading reports for the commit cfb8773 to get more accurate results

@@           Coverage Diff           @@
##             main    #2937   +/-   ##
=======================================
  Coverage   84.71%   84.72%           
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  Files         259      260    +1     
  Lines        9121     9124    +3     
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+ Hits         7727     7730    +3     
  Misses       1394     1394           

Impacted Files	Coverage Δ
...penTelemetry.Exporter.InMemory/InMemoryExporter.cs	100.00% <100.00%> (ø)
...rter.InMemory/InMemoryExporterMetricsExtensions.cs	66.66% <100.00%> (ø)
...emetry.Exporter.InMemory/InMemoryMetricExporter.cs	100.00% <100.00%> (ø)

[cijothomas]

if we (InMemoryExporter) does the clearing:

1. We are clearing something collection given to us by the user.
2. This prevents the user from doing comparison between 2 exports.

We could simply let the user do this, if they chose to. And document this in the readme.

[cijothomas]

we can add comment here.

[reyang]

May I get a summary of what's the promise from this PR?

e.g. the metrics stored in the in-memory-exporter will be available until <when?>

The reason I'm asking is:

var meterProvider = Sdk.CreateMeterProviderBuilder()
    .AddMeter(MyMeter.Name)
    .AddInMemoryExporter(collection)
    .Build();

meterProvider.Dispose();

// are the data points in `collection` still available?

[TimothyMothra]

Per SIG Meeting, will abandon this PR in favor of doc updates