fixing InMemoryExporter & Metrics bug. new class: MetricSnapshot. new ctor: InMemoryExporter(Func)

[TimothyMothra]

Bug: #2361
Supersedes: #3198.

Changes

• new class MetricSnapshot. This is used for a strategic selective copy of the minimum members to facilitate unit testing.
  • other names considered: ExportedMetric, ExportableMetricCopy, InMemoryMetricCopy, ReadOnlyMetric.
• new constructor added to InMemoryExporter to accept Func<Batch<T>, ExportResult.
  This allows the export behavior to be fully replaced and provides greater flexibility for unit tests.
• new extension methods AddInMemoryExporter(ICollection<MetricSnapshot>) added to InMemoryExporterMetricsExtensions.

Please provide a brief description of the changes here.

Metric instances are reused by design.
As a side effect, the InMemoryExporter will repeatedly export duplicates of Metric instances.
Because of this, Metrics can be modified after export. See also: #2361 (comment)

This change introduces a new class MetricSnapshot to be used in unit tests.
This class is a collection of minimum members needed for existing unit tests.
An end user has the option to use either this copy or the original Metric.

This change also introduces a new ctor public InMemoryExporter(Func<Batch<T>, ExportResult> exportFunc).
This is needed to export a type that differs from the Batch type, in this case converting Metric to MetricSnapshot.

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

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

Additional Comment

This PR keeps the existing functionality, allowing users to export Metrics via the InMemoryExporter.
We have some tests that have a dependency on the Metric.

• PrometheusSerializerTests needs Metric to test PrometheusSerializer.WriteMetric()
• OtlpMetricsExporterTests needs Batch<Metric> to test ExportMetricsServiceRequest.AddMetrics()

[TimothyMothra]

@cijothomas, this PR is meant to address your ask here: #3198 (comment)

To unblock progress - lets split PR into sub Prs.

1. Add the additional option in InMemoryExporter to use MetricCopy, leave existing functionality untouched. i.e don't change unit tests, except the one breaking due to the metric-being-updated-after-export issue.

[codecov]

Codecov Report

Merging #3266 (3427907) into main (8aa1778) will increase coverage by 0.04%.
The diff coverage is 76.31%.

@@            Coverage Diff             @@
##             main    #3266      +/-   ##
==========================================
+ Coverage   85.22%   85.27%   +0.04%     
==========================================
  Files         264      265       +1     
  Lines        9505     9541      +36     
==========================================
+ Hits         8101     8136      +35     
- Misses       1404     1405       +1     

Impacted Files	Coverage Δ
.../OpenTelemetry.Exporter.InMemory/MetricSnapshot.cs	60.00% <60.00%> (ø)
...rter.InMemory/InMemoryExporterMetricsExtensions.cs	75.00% <83.33%> (+8.33%)	⬆️
...penTelemetry.Exporter.InMemory/InMemoryExporter.cs	92.30% <100.00%> (-7.70%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	76.47% <0.00%> (+2.94%)	⬆️
...entation/ExportClient/OtlpGrpcTraceExportClient.cs	50.00% <0.00%> (+14.28%)	⬆️
...xporter.OpenTelemetryProtocol/OtlpTraceExporter.cs	59.09% <0.00%> (+22.72%)	⬆️

[cijothomas]

Left 2 comments - please address them as well.

[TimothyMothra]

@cijothomas, i just updated the changelog;

opentelemetry-dotnet/src/OpenTelemetry.Exporter.InMemory/CHANGELOG.md

Lines 5 to 7 in f2673c7

	* Adds new `AddInMemoryExporter` extension method to export `Metric` as new
	type `MetricSnapshot`.
	([#2361](https://github.com/open-telemetry/opentelemetry-dotnet/issues/2361))