[Infra] Polyfill Stopwatch.GetElapsedTime

[martincostello]

Refactoring opportunity I spotted while doing other work that relates to open-telemetry/opentelemetry-dotnet-contrib#4300.

Changes

• Add a polyfill for Stopwatch.GetElapsedTime() for netstandard2.0 and net462 for use instead of allocating Stopwatchs.
• Simplify timeout computations and duplicative code.
• Rename cur to current for readability.

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

❌ Patch coverage is 93.98496% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.75%. Comparing base (c4fa592) to head (ef3aec4).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...rc/OpenTelemetry/Internal/BatchExportTaskWorker.cs	25.00%	6 Missing ⚠️
.../OpenTelemetry/Internal/BatchExportThreadWorker.cs	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7193      +/-   ##
==========================================
+ Coverage   89.69%   89.75%   +0.05%     
==========================================
  Files         272      272              
  Lines       13434    13381      -53     
==========================================
- Hits        12050    12010      -40     
+ Misses       1384     1371      -13     

Flag	Coverage Δ
unittests-Project-Experimental	89.56% <93.98%> (+0.01%)	⬆️
unittests-Project-Stable	89.72% <93.98%> (+0.16%)	⬆️
unittests-Solution	89.62% <93.98%> (+0.17%)	⬆️
unittests-UnstableCoreLibraries-Experimental	41.34% <38.40%> (+0.02%)	⬆️

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

Files with missing lines	Coverage Δ
...tlpExporterPersistentStorageTransmissionHandler.cs	96.49% <100.00%> (+3.38%)	⬆️
...on/Transmission/OtlpExporterTransmissionHandler.cs	100.00% <100.00%> (ø)
src/OpenTelemetry/BatchExportProcessor.cs	96.15% <100.00%> (+1.32%)	⬆️
src/OpenTelemetry/CompositeProcessor.cs	95.45% <100.00%> (-0.14%)	⬇️
...nternal/PeriodicExportingMetricReaderTaskWorker.cs	75.75% <100.00%> (+0.75%)	⬆️
...ernal/PeriodicExportingMetricReaderThreadWorker.cs	78.43% <100.00%> (+2.95%)	⬆️
...emetry/Metrics/Reader/BaseExportingMetricReader.cs	90.90% <100.00%> (-1.40%)	⬇️
...nTelemetry/Metrics/Reader/CompositeMetricReader.cs	77.61% <100.00%> (+2.24%)	⬆️
...lemetry/Metrics/Reader/CompositeMetricReaderExt.cs	100.00% <100.00%> (ø)
src/OpenTelemetry/Metrics/Reader/MetricReader.cs	91.94% <100.00%> (-0.16%)	⬇️
... and 3 more

[Copilot]

Pull request overview

This PR introduces a shared Stopwatch.GetElapsedTime polyfill for older TFMs and refactors multiple timeout/elapsed-time computations to avoid allocating Stopwatch instances, plus some small readability cleanups.

Changes:

• Add src/Shared/StopwatchExtensions.cs to polyfill Stopwatch.GetElapsedTime(long) on non-NET TFMs and add a shared Stopwatch.Remaining(...) helper.
• Replace Stopwatch.StartNew() allocations with timestamp-based elapsed/remaining computations across processors/readers/workers.
• Rename loop variable cur to current in composite structures.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
test/OpenTelemetry.Tests/Metrics/MetricApiTests.cs	Uses timestamp + GetElapsedTime instead of allocating a Stopwatch for test timing output.
src/Shared/StopwatchExtensions.cs	Adds GetElapsedTime polyfill (non-NET) and a Remaining(...) helper.
src/OpenTelemetry/OpenTelemetry.csproj	Links the shared StopwatchExtensions.cs into the OpenTelemetry SDK project.
src/OpenTelemetry/Metrics/Reader/PeriodicExportingMetricReader.cs	Simplifies shutdown timeout computation using timestamp + remaining helper.
src/OpenTelemetry/Metrics/Reader/MetricReader.cs	Refactors collect timeout handling to timestamp + remaining helper and reduces duplicated logging branches.
src/OpenTelemetry/Metrics/Reader/CompositeMetricReaderExt.cs	Renames cur to current for readability.
src/OpenTelemetry/Metrics/Reader/CompositeMetricReader.cs	Refactors composite reader timeouts to timestamp-based logic and renames cur to current.
src/OpenTelemetry/Metrics/Reader/BaseExportingMetricReader.cs	Refactors shutdown timeout handling using timestamp + remaining helper.
src/OpenTelemetry/Internal/PeriodicExportingMetricReaderThreadWorker.cs	Refactors interval timeout computation to use GetElapsedTime instead of a Stopwatch instance.
src/OpenTelemetry/Internal/PeriodicExportingMetricReaderTaskWorker.cs	Same interval timeout refactor for task-based worker.
src/OpenTelemetry/Internal/BatchExportThreadWorker.cs	Refactors wait timeout logic to timestamp-based approach and simplifies Start/Shutdown.
src/OpenTelemetry/Internal/BatchExportTaskWorker.cs	Same wait timeout refactor for task-based worker; also simplifies Start/Shutdown and adjusts cancellation guards.
src/OpenTelemetry/CompositeProcessor.cs	Refactors force-flush/shutdown timeout logic and renames cur to current.
src/OpenTelemetry/BatchExportProcessor.cs	Simplifies force-flush expression body and refactors shutdown timeout computation.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/Transmission/OtlpExporterTransmissionHandler.cs	Minor simplification in request submission and refactors shutdown timeout with timestamp-based helper.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/Transmission/OtlpExporterPersistentStorageTransmissionHandler.cs	Refactors shutdown timeout computation to timestamp-based helper.
OpenTelemetry.slnx	Adds the new shared source file to the solution view.

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[martincostello]

Need to make StyleCop happy with extension somehow...