fix wrong context propagated to callback of traceContext and traceContextSync

Author: blakeroberts-wk

lib/api.dart

@@ -10,6 +10,7 @@ export 'src/api/context/context.dart'
         attach,
         Context,
         ContextKey,
+        contextFromZone,
         contextWithSpan,
         contextWithSpanContext,
         detach,

lib/src/api/open_telemetry.dart

@@ -48,7 +48,7 @@ Future<T> traceContext<T>(String name, Future<T> Function(api.Context) fn,
     bool newRoot = false,
     api.SpanKind spanKind = api.SpanKind.internal,
     List<api.SpanLink> spanLinks = const []}) async {
-  return trace(name, () => fn(context ?? api.active),
+  return trace(name, () => fn(api.contextFromZone()),
       context: context,
       tracer: tracer,
       newRoot: newRoot,
@@ -65,7 +65,7 @@ T traceContextSync<T>(String name, T Function(api.Context) fn,
     bool newRoot = false,
     api.SpanKind spanKind = api.SpanKind.internal,
     List<api.SpanLink> spanLinks = const []}) {
-  return traceSync(name, () => fn(context ?? api.active),
+  return traceSync(name, () => fn(api.contextFromZone()),
       context: context,
       tracer: tracer,
       newRoot: newRoot,