[Logging] Make TagName and TagProvider attributes work when applied to properties

[LouisDeconinck]

Fixes #6605

When a property of a [LogProperties] parameter was annotated with [TagName] or [TagProvider], the generated logging code ignored both attributes: generated tag names always used the property name and the provider method was never invoked. The parser already picked up both attributes; the emitter just never used them.

Changes

• Emitter: use each property's TagName (the custom name when [TagName] is present) when composing generated tag names. The root chain element now uses the parameter name so a {Template} binding doesn't leak into property tag names.
• Emitter: invoke property-level tag providers. The provider is called with LoggerMessageState.TagNamePrefix set to the full property path (e.g. param.Nested.Property), honoring OmitReferenceName on both the parameter ([LogProperties(OmitReferenceName = true)] drops the parameter prefix) and the property ([TagProvider(..., OmitReferenceName = true)] drops the property name itself). Properties handled by a provider no longer occupy reserved tag slots.
• Emitter: when [LogProperties(SkipNullProperties = true)] is used, a null property value skips the provider call entirely, consistent with how other null properties are skipped. The provider call is also null-guarded when the property access can produce null but the provider signature doesn't accept it.
• Attributes: removed [Conditional("CODE_GENERATION_ATTRIBUTES")] from TagNameAttribute and TagProviderAttribute so the attributes persist to metadata. This is what makes them work when the logged object is defined in a different assembly than the logging method — the same approach as [Logging] Fixes LogProperties and LogPropertyIgnore attributes to work if an object being logged resides in a different assembly than the logging method #6600 for LogPropertiesAttribute/LogPropertyIgnoreAttribute.
• Parser: made the syntax reference access null-safe in ProcessTagProviderForProperty for attributes coming from metadata.

Tests

• New runtime tests cover TagName, TagProvider (including nested and OmitReferenceName), SkipNullProperties, and the cross-assembly scenario via Microsoft.Gen.Logging.HelperLibrary.
• Updated existing expectations for the pre-existing [TagProvider] on ClassToLog.AnotherStringProperty, which is now actually invoked.
• New parser/emitter test asserts the generated source uses the custom tag name and calls the provider.

Microsoft.Gen.Logging.Unit.Tests: 368/368 pass. Microsoft.Gen.Logging.Generated.Tests: 106/106 pass (net8.0 and net10.0).

Microsoft Reviewers: Open in CodeFlow

[Copilot]

🟡 Changes recommended

Unresolved generated-string escaping, nested-prefix/nullability handling, and parameter tag-name preservation issues remain.

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Pull request overview

This PR enables property-level [TagName] and [TagProvider] support in generated logging code, including cross-assembly scenarios.

Changes:

• Emits custom property tag names and invokes property tag providers.
• Preserves attribute metadata and improves metadata parsing.
• Adds parser, runtime, nested-property, null-handling, and cross-assembly tests.

File summaries

File	Summary
test/Generators/Microsoft.Gen.Logging/Unit/ParserTests.TagProvider.cs	Tests generated property tag and provider code.
test/Generators/Microsoft.Gen.Logging/TestClasses/TagProviderExtensions.cs	Adds provider test fixtures.
test/Generators/Microsoft.Gen.Logging/TestClasses/TagAttributesOnPropertiesExtensions.cs	Adds property-attribute logging fixtures.
test/Generators/Microsoft.Gen.Logging/HelperLibrary/ProvidedPropertyTagProvider.cs	Adds a cross-assembly provider.
test/Generators/Microsoft.Gen.Logging/HelperLibrary/ProvidedProperty.cs	Adds a cross-assembly test model.
test/Generators/Microsoft.Gen.Logging/HelperLibrary/ObjectToLogWithTagAttributes.cs	Adds cross-assembly attributed objects.
test/Generators/Microsoft.Gen.Logging/Generated/TagProviderTests.cs	Updates provider expectations.
test/Generators/Microsoft.Gen.Logging/Generated/LogPropertiesTests.cs	Adds runtime coverage for property attributes.
src/Libraries/Microsoft.Extensions.Telemetry.Abstractions/Logging/TagProviderAttribute.cs	Preserves provider metadata.
src/Libraries/Microsoft.Extensions.Telemetry.Abstractions/Logging/TagNameAttribute.cs	Preserves tag-name metadata.
src/Generators/Microsoft.Gen.Logging/Parsing/Parser.TagProvider.cs	Handles metadata-only provider attributes safely.
src/Generators/Microsoft.Gen.Logging/Model/LoggingMethodParameterExtensions.cs	Builds property traversal roots.
src/Generators/Microsoft.Gen.Logging/Emission/Emitter.Method.cs	Emits property tags and provider calls.

Review details

Suppressed comments (2)

src/Generators/Microsoft.Gen.Logging/Emission/Emitter.Method.cs:545

• This null guard uses member.IsNullable rather than the provider method's second-parameter nullability. ValidateTagProvider permits a nullable provider argument for a non-nullable property, so when a parent in the property chain is null and SkipNullProperties is false, a valid provider such as Provide(..., Child? value) is incorrectly skipped instead of being invoked with null. Track the provider parameter's nullability and guard only when it cannot accept null; keep the SkipNullProperties guard independent.

                        var canProduceNull = member.PotentiallyNull || propertyChain.Any(static x => x.PotentiallyNull);
                        if (canProduceNull && (!member.IsNullable || p.SkipNullProperties))

src/Generators/Microsoft.Gen.Logging/Model/LoggingMethodParameterExtensions.cs:20

• This discards an explicit [TagName] on a [LogProperties] parameter. Before this change, parameter.TagName was the root of every property path; only the synthesized template name needs to be avoided here. For [TagName("payload")] [LogProperties] MyType value, this changes payload.Property to value.Property. Preserve the custom tag name unless parameter.UsedAsTemplate is true.

            TagName = parameter.ParameterName,

• Files reviewed: 11/13 changed files
• Comments generated: 2
• Review effort level: Lite

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[LouisDeconinck]

Addressed the latest review feedback in e06c48c:

• Escaping: tag names emitted into generated string literals now go through EscapeMessageString (SymbolDisplay.FormatLiteral), including the TagNamePrefix assignment for property-level providers and the tag name literals emitted for TagArray/ClassifiedTagArray/AddTag/AddClassifiedTag calls. Custom [TagName] values containing quotes, backslashes, etc. now produce compilable generated code.
• Transitive coverage: added runtime tests for a [TagProvider] on a transitively nested leaf property (LogPropertiesSupportsTagProviderOnTransitivelyNestedProperty[WhenOmittingParameterName]), covering all four combinations of parameter-level LogProperties(OmitReferenceName = true) and property-level TagProvider(OmitReferenceName = true).
• Unit tests: TagNameAndTagProviderOnPropertiesAreEscapedInGeneratedSource verifies that tag names containing a quote and a backslash are escaped in the emitted literals and that the generated source compiles, and TagProviderOnTransitivelyNestedProperty asserts the composed TagNamePrefix values for the nested OmitReferenceName combinations.

All Microsoft.Gen.Logging tests pass: 370 unit + 108 generated (net8.0/net9.0/net10.0).