[Instana] Use System.Text.Json

[martincostello]

Builds on top of #4153.

Changes

Use System.Text.Json to serialize spans.

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 96.61017% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.83%. Comparing base (077386e) to head (82389a5).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...er.Instana/Implementation/InstanaSpanSerializer.cs	95.55%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4293      +/-   ##
==========================================
- Coverage   75.91%   75.83%   -0.09%     
==========================================
  Files         466      466              
  Lines       18549    18475      -74     
==========================================
- Hits        14081    14010      -71     
+ Misses       4468     4465       -3     

Flag	Coverage Δ
unittests-Exporter.Instana	92.00% <96.61%> (+0.10%)	⬆️
unittests-Resources.Azure	88.31% <ø> (ø)
unittests-Resources.Gcp	71.42% <ø> (ø)

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

Files with missing lines	Coverage Δ
...metry.Exporter.Instana/Implementation/Transport.cs	90.62% <100.00%> (+1.56%)	⬆️
...er.Instana/Implementation/InstanaSpanSerializer.cs	91.25% <95.55%> (-0.61%)	⬇️

... and 5 files with indirect coverage changes

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[Copilot]

Pull request overview

This PR updates the Instana exporter to use System.Text.Json/Utf8JsonWriter for span payload serialization, improving JSON correctness (escaping) and aligning with the broader move away from manual JSON building.

Changes:

• Replace manual JSON string writing with Utf8JsonWriter-based serialization for Instana spans and bundle payloads.
• Add/adjust unit tests to validate correct JSON encoding of quotes and newlines in exported span fields.
• Add conditional System.Text.Json package reference for TFMs below net8.0 and record the change in the exporter changelog.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/OpenTelemetry.Exporter.Instana.Tests/InstanaSpanSerializerTests.cs	Switch test serialization path to Utf8JsonWriter and continue validating round-trip JSON deserialization.
test/OpenTelemetry.Exporter.Instana.Tests/InstanaExporterTests.cs	Add test coverage ensuring exporter JSON payload correctly encodes quotes/newlines and event/tag content.
src/OpenTelemetry.Exporter.Instana/OpenTelemetry.Exporter.Instana.csproj	Add conditional System.Text.Json reference for non-net8.0+ TFMs.
src/OpenTelemetry.Exporter.Instana/Implementation/Transport.cs	Serialize the bundle request body using Utf8JsonWriter instead of manual string writing.
src/OpenTelemetry.Exporter.Instana/Implementation/InstanaSpanSerializer.cs	Rework span serialization to Utf8JsonWriter to ensure safe string encoding.
src/OpenTelemetry.Exporter.Instana/CHANGELOG.md	Document the serializer change under Unreleased.

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