Send App Id when Firestore SDK delivers heartbeat (#1055)

Send GMP App Id along with Firestore requests for desktop platform similar to for [Apple platform](https://github.com/firebase/firebase-ios-sdk/blob/eb30b6fdd809cbfe3a9eb7f3ccf1dd46326bef2d/Firestore/core/src/remote/firebase_metadata_provider_apple.mm#L66). 
This is required for the backend to process heartbeat properly for desktop usage.

Author: chkuang-g

firestore/src/main/create_firebase_metadata_provider_desktop.cc

@@ -23,8 +23,9 @@ namespace firestore {
 
 using remote::FirebaseMetadataProvider;
 
-std::unique_ptr<FirebaseMetadataProvider> CreateFirebaseMetadataProvider(App&) {
-  return absl::make_unique<FirebaseMetadataProviderCpp>();
+std::unique_ptr<FirebaseMetadataProvider> CreateFirebaseMetadataProvider(
+    App& app) {
+  return absl::make_unique<FirebaseMetadataProviderCpp>(app);
 }
 
 }  // namespace firestore

firestore/src/main/firebase_metadata_provider_desktop.cc

@@ -22,6 +22,9 @@
 namespace firebase {
 namespace firestore {
 
+FirebaseMetadataProviderCpp::FirebaseMetadataProviderCpp(const App& app)
+    : gmp_app_id_(app.options().app_id()) {}
+
 void FirebaseMetadataProviderCpp::UpdateMetadata(grpc::ClientContext& context) {
   HeartbeatInfo::Code heartbeat = HeartbeatInfo::GetHeartbeatCode("fire-fst");
 
@@ -34,6 +37,10 @@ void FirebaseMetadataProviderCpp::UpdateMetadata(grpc::ClientContext& context) {
   }
 
   context.AddMetadata(kXFirebaseClientHeader, App::GetUserAgent());
+
+  if (!gmp_app_id_.empty()) {
+    context.AddMetadata(kXFirebaseGmpIdHeader, gmp_app_id_);
+  }
 }
 
 }  // namespace firestore

firestore/src/main/firebase_metadata_provider_desktop.h

@@ -17,7 +17,10 @@
 #ifndef FIREBASE_FIRESTORE_SRC_MAIN_FIREBASE_METADATA_PROVIDER_DESKTOP_H_
 #define FIREBASE_FIRESTORE_SRC_MAIN_FIREBASE_METADATA_PROVIDER_DESKTOP_H_
 
+#include <string>
+
 #include "Firestore/core/src/remote/firebase_metadata_provider.h"
+#include "app/src/include/firebase/app.h"
 
 #if defined(__ANDROID__)
 #error "This header should not be used on Android."
@@ -29,7 +32,12 @@ namespace firestore {
 
 class FirebaseMetadataProviderCpp : public remote::FirebaseMetadataProvider {
  public:
+  explicit FirebaseMetadataProviderCpp(const App& app);
+
   void UpdateMetadata(grpc::ClientContext& context) override;
+
+ private:
+  std::string gmp_app_id_;
 };
 
 }  // namespace firestore