Ensure template is not sent for Structured logs with no parameters

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Fixes
 
+- Templates are no longer sent with Structured Logs that have no parameters ([#4544](https://github.com/getsentry/sentry-dotnet/pull/4544))
 - Upload linked PDBs to fix non-IL-stripped symbolication for iOS ([#4527](https://github.com/getsentry/sentry-dotnet/pull/4527))
 - In MAUI Android apps, generate and inject UUID to APK and upload ProGuard mapping to Sentry with the UUID ([#4532](https://github.com/getsentry/sentry-dotnet/pull/4532))
 - Fixed WASM0001 warning when building Blazor WebAssembly projects ([#4519](https://github.com/getsentry/sentry-dotnet/pull/4519))

src/Sentry/SentryLog.cs

@@ -224,7 +224,9 @@ internal void WriteTo(Utf8JsonWriter writer, IDiagnosticLogger? logger)
         writer.WritePropertyName("attributes");
         writer.WriteStartObject();
 
-        if (Template is not null)
+        // the SDK MUST NOT attach a sentry.message.template attribute if there are no parameters
+        // https://develop.sentry.dev/sdk/telemetry/logs/#default-attributes
+        if (Template is not null && !Parameters.IsDefaultOrEmpty)
         {
             SentryAttributeSerializer.WriteStringAttribute(writer, "sentry.message.template", Template);
         }

test/Sentry.Tests/SentryLogTests.cs

@@ -67,6 +67,30 @@ public void Protocol_Default_VerifyAttributes()
         notFound.Should().BeNull();
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void WriteTo_NoParameters_NoTemplate(bool hasParameters)
+    {
+        // Arrange
+        ImmutableArray<KeyValuePair<string, object>> parameters = hasParameters
+            ? [new KeyValuePair<string, object>("param", "params")]
+            : [];
+        var log = new SentryLog(Timestamp, TraceId, SentryLogLevel.Debug, "message")
+        {
+            Template = "template",
+            Parameters = parameters,
+            ParentSpanId = ParentSpanId,
+        };
+
+        // Act
+        var document = log.ToJsonDocument(static (obj, writer, logger) => obj.WriteTo(writer, logger), _output);
+        var attributes = document.RootElement.GetProperty("attributes");
+
+        // Assert
+        attributes.TryGetProperty("sentry.message.template", out _).Should().Be(hasParameters);
+    }
+
     [Fact]
     public void WriteTo_Envelope_MinimalSerializedSentryLog()
     {