Add deprecation for context.root

lib/src/api/context/context.dart

@@ -28,6 +28,8 @@ abstract class Context {
   /// Only use this context if you are certain you need to disregard the
   /// current [Context].  For example, when instrumenting an asynchronous
   /// event handler which may fire while an unrelated [Context] is "current".
+  @Deprecated(
+      'We are planning to remove this in the future, please use Context.current instead.')
   static Context get root => _contextManager.root;
 
   /// Returns a key to be used to read and/or write values to a context.

lib/src/api/context/context_manager.dart

@@ -4,6 +4,8 @@
 import 'package:opentelemetry/api.dart';
 
 abstract class ContextManager {
+  @Deprecated(
+      'We are planning to remove this in the future, please use Context.current instead.')
   Context get root;
 
   Context get active;

lib/src/sdk/context/zone_context.dart

@@ -47,6 +47,8 @@ class ZoneContext implements Context {
   /// Only use this context if you are certain you need to disregard the
   /// current [ZoneContext].  For example, when instrumenting an asynchronous
   /// event handler which may fire while an unrelated [ZoneContext] is "current".
+  @Deprecated(
+      'We are planning to remove this in the future, please use Context.current instead.')
   static ZoneContext get root => ZoneContext._(Zone.root);
 
   /// Returns a key to be used to read and/or write values to a context.

lib/src/sdk/context/zone_context_manager.dart

@@ -8,6 +8,8 @@ class ZoneContextManager implements ContextManager {
   @override
   Context get active => ZoneContext.current;
 
+  @Deprecated(
+      'We are planning to remove this in the future, please use Context.current instead.')
   @override
   Context get root => ZoneContext.root;
 }