[Tests] Fix flaky tests

[martincostello]

Changes

Refactor two different test classes to try and avoid flakiness in CI.

Also fixes the IDE suggestions from Visual Studio while I was touching the files.

First:

[xUnit.net 00:00:04.36]     OpenTelemetry.Metrics.Tests.MetricExemplarTests.TestExemplarsExponentialHistogram(temporality: Cumulative) [FAIL]
[xUnit.net 00:00:04.36]       Assert.NotEqual() Failure: Values are equal
[xUnit.net 00:00:04.36]       Expected: Not 00000000000000000000000000000000
[xUnit.net 00:00:04.36]       Actual:       00000000000000000000000000000000
[xUnit.net 00:00:04.36]       Stack Trace:
[xUnit.net 00:00:04.36]         /home/runner/work/opentelemetry-dotnet/opentelemetry-dotnet/test/OpenTelemetry.Tests/Metrics/MetricExemplarTests.cs(844,0): at OpenTelemetry.Metrics.Tests.MetricExemplarTests.ValidateExemplars(IReadOnlyList`1 exemplars, DateTimeOffset startTime, DateTimeOffset endTime, IEnumerable`1 measurementValues, Func`2 getExemplarValueFunc)
[xUnit.net 00:00:04.36]         /home/runner/work/opentelemetry-dotnet/opentelemetry-dotnet/test/OpenTelemetry.Tests/Metrics/MetricExemplarTests.cs(670,0): at OpenTelemetry.Metrics.Tests.MetricExemplarTests.<TestExemplarsExponentialHistogram>g__ValidateSecondPhase|6_2(String instrumentName, MetricReaderTemporalityPreference temporality, DateTime testStartTime, List`1 exportedItems, ValueTuple`2[] firstMeasurementValues, ValueTuple`2[] secondMeasurementValues)
[xUnit.net 00:00:04.36]         /home/runner/work/opentelemetry-dotnet/opentelemetry-dotnet/test/OpenTelemetry.Tests/Metrics/MetricExemplarTests.cs(624,0): at OpenTelemetry.Metrics.Tests.MetricExemplarTests.TestExemplarsExponentialHistogram(MetricReaderTemporalityPreference temporality)
[xUnit.net 00:00:04.36]            at InvokeStub_MetricExemplarTests.TestExemplarsExponentialHistogram(Object, Span`1)
[xUnit.net 00:00:04.36]            at System.Reflection.MethodBaseInvoker.InvokeWithOneArg(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

Second:

[xUnit.net 00:00:02.95]     OpenTelemetry.Metrics.Tests.MetricPointReclaimTests.MeasurementsAreAggregatedEvenAfterTheyAreDropped(emitMetricWithNoDimension: True) [FAIL]
[xUnit.net 00:00:02.95]       Assert.Equal() Failure: Values differ
[xUnit.net 00:00:02.95]       Expected: 3680
[xUnit.net 00:00:02.95]       Actual:   3670
[xUnit.net 00:00:02.96]       Stack Trace:
[xUnit.net 00:00:02.96]         Metrics\MetricPointReclaimTests.cs(174,0): at OpenTelemetry.Metrics.Tests.MetricPointReclaimTests.MeasurementsAreAggregatedEvenAfterTheyAreDropped(Boolean emitMetricWithNoDimension)

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.85%. Comparing base (d855a7a) to head (bd6baf7).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6734   +/-   ##
=======================================
  Coverage   86.85%   86.85%           
=======================================
  Files         258      258           
  Lines       11990    11990           
=======================================
  Hits        10414    10414           
  Misses       1576     1576           

Flag	Coverage Δ
unittests-Project-Experimental	86.67% <ø> (-0.03%)	⬇️
unittests-Project-Stable	86.46% <ø> (-0.26%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[Copilot]

Pull request overview

This PR addresses flaky test failures in two test classes by improving timing reliability and refactoring code for better clarity. The changes focus on ensuring proper resource cleanup timing and verifying that ForceFlush operations complete successfully.

• Increased timeout for flush operations from 10 seconds to 15 seconds to accommodate slower CI environments
• Wrapped MeterProvider instances in using blocks to ensure disposal happens after final assertions
• Added assertions to verify ForceFlush operations succeed rather than silently ignoring failures

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
MetricPointReclaimTests.cs	Refactored to ensure MeterProvider disposal occurs after all measurements are validated, converted variables to constants
MetricExemplarTests.cs	Increased flush timeout and added assertions to verify ForceFlush success, simplified conditional expressions and loop variable destructuring

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