Handle case where SetException throws (#5291)

tracer/src/Datadog.Trace/Span.cs

@@ -405,17 +405,25 @@ internal void SetExceptionTags(Exception exception)
         {
             if (exception != null)
             {
-                // for AggregateException, use the first inner exception until we can support multiple errors.
-                // there will be only one error in most cases, and even if there are more and we lose
-                // the other ones, it's still better than the generic "one or more errors occurred" message.
-                if (exception is AggregateException aggregateException && aggregateException.InnerExceptions.Count > 0)
+                try
                 {
-                    exception = aggregateException.InnerExceptions[0];
-                }
+                    // for AggregateException, use the first inner exception until we can support multiple errors.
+                    // there will be only one error in most cases, and even if there are more and we lose
+                    // the other ones, it's still better than the generic "one or more errors occurred" message.
+                    if (exception is AggregateException aggregateException && aggregateException.InnerExceptions.Count > 0)
+                    {
+                        exception = aggregateException.InnerExceptions[0];
+                    }
 
-                SetTag(Trace.Tags.ErrorMsg, exception.Message);
-                SetTag(Trace.Tags.ErrorStack, exception.ToString());
-                SetTag(Trace.Tags.ErrorType, exception.GetType().ToString());
+                    SetTag(Trace.Tags.ErrorMsg, exception.Message);
+                    SetTag(Trace.Tags.ErrorType, exception.GetType().ToString());
+                    SetTag(Trace.Tags.ErrorStack, exception.ToString());
+                }
+                catch (Exception ex)
+                {
+                    // We have found rare cases where exception.ToString() throws an exception, such as in a FileNotFoundException
+                    Log.Warning(ex, "Error setting exception tags on span {SpanId} in trace {TraceId}", SpanId, TraceId);
+                }
             }
         }