Reduce allocations in converting to message

Author: andrewlock

tracer/src/Datadog.Trace/LibDatadog/Error.cs

@@ -5,6 +5,7 @@
 
 #nullable enable
 
+using System;
 using System.Runtime.InteropServices;
 using System.Text;
 using Datadog.Trace.Logging;
@@ -25,8 +26,6 @@ internal readonly struct Error
 
     public LibDatadogException ToException()
     {
-        var messageBytes = Message.ToByteArray();
-        var message = Encoding.UTF8.GetString(messageBytes);
-        return new LibDatadogException(message);
+        return new LibDatadogException(Message.ToUtf8String());
     }
 }

tracer/src/Datadog.Trace/LibDatadog/FFIVec.cs

@@ -7,6 +7,7 @@
 
 using System;
 using System.Runtime.InteropServices;
+using System.Text;
 
 namespace Datadog.Trace.LibDatadog;
 
@@ -17,10 +18,16 @@ internal readonly struct FFIVec
     internal readonly nuint Length;
     internal readonly nuint Capacity;
 
-    public byte[] ToByteArray()
+    public string ToUtf8String()
     {
-        var bytes = new byte[Length];
-        Marshal.Copy(Data, bytes, 0, (int)Length);
-        return bytes;
+        unsafe
+        {
+#if NETCOREAPP
+            var messageBytes = new ReadOnlySpan<byte>(Data.ToPointer(), (int)Length);
+            return Encoding.UTF8.GetString(messageBytes);
+#else
+            return Encoding.UTF8.GetString((byte*)Data.ToPointer(), (int)Length);
+#endif
+        }
     }
 }