Avoid string allocation in WriteTo when possible.

System.Text.Json.JsonProperty.WriteTo uses get_Name, calling
JsonElement.GetPropertyName() which would allocate a string.

Use ReadOnlySpan<byte> from the underlying UTF8 json, when possible,
by adding helper methods into JsonDocument & JsonElement.

Fix dotnet#88767

src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonDocument.cs

@@ -283,6 +283,29 @@ private ReadOnlyMemory<byte> GetPropertyRawValue(int valueIndex)
                 : JsonReaderHelper.TranscodeHelper(segment);
         }
 
+        internal ReadOnlySpan<byte> GetUtf8Span(int index, JsonTokenType expectedType)
+        {
+            CheckNotDisposed();
+
+            DbRow row = _parsedData.Get(index);
+
+            JsonTokenType tokenType = row.TokenType;
+
+            if (tokenType == JsonTokenType.Null)
+            {
+                return default;
+            }
+
+            CheckExpectedType(expectedType, tokenType);
+
+            ReadOnlySpan<byte> data = _utf8Json.Span;
+            ReadOnlySpan<byte> segment = data.Slice(row.Location, row.SizeOrLength);
+
+            return row.HasComplexChildren
+                ? JsonReaderHelper.GetUnescapedSpan(segment)
+                : segment;
+        }
+
         internal bool TextEquals(int index, ReadOnlySpan<char> otherText, bool isPropertyName)
         {
             CheckNotDisposed();
@@ -363,6 +386,12 @@ internal string GetNameOfPropertyValue(int index)
             return GetString(index - DbRow.Size, JsonTokenType.PropertyName)!;
         }
 
+        internal ReadOnlySpan<byte> GetNameOfPropertyValueAsUtf8Span(int index)
+        {
+            // The property name is one row before the property value
+            return GetUtf8Span(index - DbRow.Size, JsonTokenType.PropertyName)!;
+        }
+
         internal bool TryGetValue(int index, [NotNullWhen(true)] out byte[]? value)
         {
             CheckNotDisposed();

src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonElement.cs

@@ -1163,6 +1163,13 @@ internal string GetPropertyName()
             return _parent.GetNameOfPropertyValue(_idx);
         }
 
+        internal ReadOnlySpan<byte> GetPropertyNameAsUtf8Span()
+        {
+            CheckValidInstance();
+
+            return _parent.GetNameOfPropertyValueAsUtf8Span(_idx);
+        }
+
         /// <summary>
         ///   Gets the original input data backing this value, returning it as a <see cref="string"/>.
         /// </summary>

src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonProperty.cs

@@ -117,7 +117,15 @@ public void WriteTo(Utf8JsonWriter writer)
                 ThrowHelper.ThrowArgumentNullException(nameof(writer));
             }
 
-            writer.WritePropertyName(Name);
+            if (_name is null)
+            {
+                writer.WritePropertyName(Value.GetPropertyNameAsUtf8Span());
+            }
+            else
+            {
+                writer.WritePropertyName(_name);
+            }
+
             Value.WriteTo(writer);
         }