feat: add array support for custom attributes in .NET Agent API

[tippmar-nr]

Summary

This PR adds comprehensive array support for custom attributes in the New Relic .NET Agent API, allowing developers to pass array values to AddCustomAttribute() that are properly serialized as JSON arrays.

Changes Made

Core Implementation

• Enhanced JSON serialization in JsonSerializerHelpers.cs:
  • Added support for IEnumerable types (arrays, lists, etc.)
  • Proper null filtering - null elements are excluded from serialized arrays
  • Empty arrays and null-only arrays are treated as null (not serialized)
  • Maintains existing conversion logic for individual array elements
  • Refactored to use single WriteValue method for all value types

Comprehensive Testing

Unit Tests (Core.UnitTest)

• Added JsonSerializerHelpersTests covering all array scenarios
• Tests for string arrays, int arrays, bool arrays, and Lists
• Validates null filtering and empty array handling
• Ensures mixed null arrays properly exclude null elements

Integration Tests

• Complete end-to-end testing through New Relic .NET Agent API
• Dual platform support: Both .NET Framework (WebAPI) and .NET Core applications
• Test applications with array attribute endpoints:
  • CustomArrayAttributes - Tests string, int, bool arrays
  • CustomEmptyArrayAttributes - Tests empty/null-only array handling
  • CustomArrayWithNulls - Tests null filtering within arrays
• Comprehensive validation: Arrays appear correctly in both transaction traces and transaction events
• Reliable test execution: Uses transaction events for multi-endpoint test scenarios

API Usage

Developers can now use array values with the New Relic API:

// All of these now work correctly
NewRelic.Api.Agent.NewRelic.GetAgent().CurrentTransaction.AddCustomAttribute("colors", new[] { "red", "green", "blue" });
NewRelic.Api.Agent.NewRelic.GetAgent().CurrentTransaction.AddCustomAttribute("numbers", new[] { 1, 2, 3, 4, 5 });  
NewRelic.Api.Agent.NewRelic.GetAgent().CurrentTransaction.AddCustomAttribute("flags", new[] { true, false, true });
NewRelic.Api.Agent.NewRelic.GetAgent().CurrentTransaction.AddCustomAttribute("items", new List<string> { "item1", "item2" });

// Null handling
NewRelic.Api.Agent.NewRelic.GetAgent().CurrentTransaction.AddCustomAttribute("filtered", new object[] { "first", null, "third" });
// Results in: ["first", "third"] - nulls are filtered out

// Empty arrays are not serialized (treated as null)
NewRelic.Api.Agent.NewRelic.GetAgent().CurrentTransaction.AddCustomAttribute("empty", new string[] { }); // Not written

Behavior

• Arrays serialize as JSON arrays: ["red", "green", "blue"]
• Null elements filtered: ["first", null, "third"] → ["first", "third"]
• Empty/null-only arrays skipped: Not included in serialized output
• Type preservation: Uses existing element conversion logic
• Framework support: Works with arrays, Lists, and any IEnumerable

Testing Results

• ✅ 8/8 integration tests passing (both .NET Framework and .NET Core)
• ✅ All unit tests passing for JSON serialization
• ✅ End-to-end validation through actual New Relic agent instrumentation
• ✅ Transaction traces and events contain properly formatted JSON arrays

Verified correct behavior in the New Relic UI:

Backward Compatibility

• ✅ No breaking changes - existing functionality unchanged
• ✅ Pure addition - only adds new array handling capability
• ✅ Same API surface - no changes to public interfaces

This feature enables developers to provide richer telemetry data by including array-based custom attributes that are properly structured and queryable in New Relic.

[tippmar-nr]

✅ Review comments addressed: Reverted JsonSerializerHelpers switch cases to use discard pattern (_) as originally implemented, fixing the inconsistent variable naming from the ReSharper refactor.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.68%. Comparing base (1ccc871) to head (69c27de).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3456      +/-   ##
==========================================
+ Coverage   81.64%   81.68%   +0.04%     
==========================================
  Files         508      508              
  Lines       33944    33963      +19     
  Branches     3999     4003       +4     
==========================================
+ Hits        27712    27743      +31     
+ Misses       5273     5264       -9     
+ Partials      959      956       -3     

Flag	Coverage Δ
Agent	82.67% <100.00%> (+0.04%)	⬆️
Profiler	71.75% <ø> (ø)

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

Files with missing lines	Coverage Δ
...Agent/Core/JsonConverters/JsonSerializerHelpers.cs	100.00% <100.00%> (+31.34%)	⬆️

... and 2 files with indirect coverage changes

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