Use ValueStringBuilder in AspNetCoreResourceNameHelper on all TFMs

Replace StringBuilderCache fallback with ValueStringBuilder and
stackalloc on all TFMs. AppendAsLowerInvariant lowercases each
segment inline, avoiding the extra string allocation from the
previous .ToLowerInvariant() call on the final result. Add a
string? overload to ValueStringBuilder.AppendAsLowerInvariant to
avoid needing explicit .AsSpan() at each call site.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: andrewlock

tracer/src/Datadog.Trace/DiagnosticListeners/AspNetCoreResourceNameHelper.cs

@@ -11,7 +11,6 @@
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
-using System.Text;
 using Datadog.Trace.DuckTyping;
 using Datadog.Trace.SourceGenerators;
 using Datadog.Trace.Util;
@@ -128,14 +127,9 @@ internal static string SimplifyRoutePattern(
                     + (string.IsNullOrEmpty(actionName) ? 0 : Math.Max(actionName!.Length - 6, 0)) // "action".Length
                     + 1; // '/' prefix
 
-#if NETCOREAPP
         var sb = maxSize < 512
                      ? new ValueStringBuilder(stackalloc char[512])
                      : new ValueStringBuilder(); // too big to use stackallocation, so use array builder
-#else
-        // In .NET Core 2.1, the ValueStringBuilder doesn't actually improve anything
-        var sb = StringBuilderCache.Acquire(maxSize);
-#endif
 
         foreach (var pathSegment in routePattern.PathSegments)
         {
@@ -227,23 +221,13 @@ internal static string SimplifyRoutePattern(
             }
         }
 
-#if NETCOREAPP
         if (sb.Length <= 1)
         {
             sb.Dispose();
             return "/";
         }
 
         return sb.ToString();
-#else
-        if (sb.Length <= 1)
-        {
-            StringBuilderCache.Release(sb);
-            return "/";
-        }
-
-        return StringBuilderCache.GetStringAndRelease(sb).ToLowerInvariant();
-#endif
     }
 
     internal static string SimplifyRouteTemplate(
@@ -261,14 +245,9 @@ internal static string SimplifyRouteTemplate(
                     + (string.IsNullOrEmpty(actionName) ? 0 : Math.Max(actionName!.Length - 6, 0)) // "action".Length
                     + 1; // '/' prefix
 
-#if NETCOREAPP
         var sb = maxSize < 512
                      ? new ValueStringBuilder(stackalloc char[512])
                      : new ValueStringBuilder(); // too big to use stackallocation, so use array builder
-#else
-        // In .NET Core 2.1, the ValueStringBuilder doesn't actually improve anything
-        var sb = StringBuilderCache.Acquire(maxSize);
-#endif
 
         // Remove the boxing of the enumerator
         // In all versions of .NET, this is implemented as a List<TemplateSegment>
@@ -367,23 +346,13 @@ internal static string SimplifyRouteTemplate(
         }
 
         // We never added anything, or we just added the first `/`, no need for explicit ToString()
-#if NETCOREAPP
         if (sb.Length <= 1)
         {
             sb.Dispose();
             return "/";
         }
 
         return sb.ToString();
-#else
-        if (sb.Length <= 1)
-        {
-            StringBuilderCache.Release(sb);
-            return "/";
-        }
-
-        return StringBuilderCache.GetStringAndRelease(sb).ToLowerInvariant();
-#endif
     }
 
     [TestingAndPrivateOnly]
@@ -412,10 +381,5 @@ internal static bool IsIdentifierSegment(object? value, out string? valueAsStrin
 
         return UriHelpers.IsIdentifierSegment(valueAsString, 0, valueAsString.Length);
     }
-
-#if !NETCOREAPP
-    // .NET Core 2.1 helper which doesn't _actually_ append as lower invariant, and just does it all at the end instead
-    private static void AppendAsLowerInvariant(this StringBuilder sb, string? value) => sb.Append(value);
-#endif
 }
 #endif

tracer/src/Datadog.Trace/Util/ValueStringBuilder.cs

@@ -232,6 +232,15 @@ public Span<char> AppendSpan(int length)
             return _chars.Slice(origPos, length);
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void AppendAsLowerInvariant(string? value)
+        {
+            if (value is not null)
+            {
+                AppendAsLowerInvariant(value.AsSpan());
+            }
+        }
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void AppendAsLowerInvariant(scoped ReadOnlySpan<char> value)
         {