Port Dispose method changes from elasticsearch-net (#14)

Author: stevejgordon

src/Elastic.Transport/Diagnostics/Diagnostic.cs

@@ -25,6 +25,7 @@ internal class Diagnostic<TState, TStateEnd> : Activity, IDisposable
 		private readonly DiagnosticSource _source;
 		private TStateEnd _endState;
 		private readonly bool _default;
+		private bool _disposed;
 
 		private Diagnostic() : base("__NOOP__") => _default = true;
 
@@ -45,8 +46,19 @@ internal set
 			}
 		}
 
+		protected override void Dispose(bool disposing)
+		{
+			if (_disposed) return;
+
+			if (disposing)
+			{
+				//_source can be null if Default instance
+				_source?.StopActivity(SetEndTime(DateTime.UtcNow), EndState);
+			}
+
+			_disposed = true;
 
-		//_source can be null if Default instance
-		protected override void Dispose(bool disposing) => _source?.StopActivity(SetEndTime(DateTime.UtcNow), EndState);
+			base.Dispose(disposing);
+		}
 	}
 }