refactor JK to function as killswitch for easier scalability

Summary:
Simple refactor - so JK lives in `updateFromVC` which will limit the usage of having to wrap
```
    if (JustKnobs::eval('meta_cp/www:enable_user_id_ctx_prop'))
```

in the instrumentation points

Reviewed By: aknott

Differential Revision: D69260239

fbshipit-source-id: ba134564e6bc221f792e22b12b0a88cd82915b0f

third-party/thrift/src/thrift/lib/hack/src/ThriftContextPropState.php

@@ -107,6 +107,14 @@ public static function initFromString(?string $s)[defaults]: void {
   }
 
   public static function updateFromVC(?IViewerContextBase $vc): void {
+    if ($vc is null) {
+      return;
+    }
+
+    if (!JustKnobs::eval('meta_cp/www:enable_user_id_ctx_prop')) {
+      return;
+    }
+
     if ($vc is IFBViewerContext) {
       self::updateFBUserIdFromVC($vc);
     } else if ($vc is IIGViewerContext) {

third-party/thrift/src/thrift/lib/hack/src/__tests__/ThriftContextPropStateTest.php

@@ -147,6 +147,7 @@ public function testInitialization()[defaults]: void {
   }
 
   public async function testInitializationWithVC()[defaults]: Awaitable<void> {
+    MockJustKnobs::setBool('meta_cp/www:enable_user_id_ctx_prop', true);
     $tfm = ThriftFrameworkMetadata::withDefaultValues();
     $tfm->baggage = ContextProp\Baggage::withDefaultValues();
     $tfm->baggage->user_ids = ContextProp\UserIds::fromShape(
@@ -175,6 +176,7 @@ public function testInitialization()[defaults]: void {
 
   public async function testInitializationWithBothTFMandVC(
   )[defaults]: Awaitable<void> {
+    MockJustKnobs::setBool('meta_cp/www:enable_user_id_ctx_prop', true);
     $tfm = ThriftFrameworkMetadata::withDefaultValues();
     $tfm->baggage = ContextProp\Baggage::withDefaultValues();
     $tfm->baggage->user_ids = ContextProp\UserIds::fromShape(
@@ -201,6 +203,7 @@ public function testInitialization()[defaults]: void {
 
   public async function testInitializationUserIdsNotOverwritten(
   )[defaults]: Awaitable<void> {
+    MockJustKnobs::setBool('meta_cp/www:enable_user_id_ctx_prop', true);
     $tfm = ThriftFrameworkMetadata::withDefaultValues();
     $tfm->baggage = ContextProp\Baggage::withDefaultValues();
     $tfm->baggage->user_ids = ContextProp\UserIds::fromShape(