Sets LogEntry.TraceSampled to true the entry is traced.

Fixes googleapis#5307

apis/Google.Cloud.Diagnostics.AspNetCore/Google.Cloud.Diagnostics.AspNetCore/Logging/GoogleLogger.cs

@@ -276,6 +276,7 @@ private void SetTraceAndSpanIfAny(LogEntry entry)
             if ((TraceContextForLogEntry.FromGoogleTrace() ?? TraceContextForLogEntry.FromExternalTrace(_serviceProvider)) is TraceContextForLogEntry trace)
             {
                 entry.Trace = _traceTarget.GetFullTraceName(trace.TraceId);
+                entry.TraceSampled = true;
                 entry.SpanId = trace.SpanId;
             }
         }