Firestore: Include a trailing /documents on root resource paths (#4352)

This is required for v1 and accepted in v1beta1.

Port of googleapis/nodejs-firestore@52c7381

Author: mikelehen

google-cloud-clients/google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreImpl.java

@@ -398,7 +398,7 @@ boolean areTimestampsInSnapshotsEnabled() {
 
   /** Returns the name of the Firestore project associated with this client. */
   String getDatabaseName() {
-    return databasePath.toString();
+    return databasePath.getDatabaseName().toString();
   }
 
   /** Returns the underlying RPC client. */

google-cloud-clients/google-cloud-firestore/src/main/java/com/google/cloud/firestore/ResourcePath.java

@@ -127,9 +127,8 @@ String getPath() {
    */
   String getName() {
     String path = getPath();
-
     if (path.isEmpty()) {
-      return getDatabaseName().toString();
+      return getDatabaseName() + "/documents";
     } else {
       return getDatabaseName() + "/documents/" + getPath();
     }

google-cloud-clients/google-cloud-firestore/src/test/java/com/google/cloud/firestore/LocalFirestoreHelper.java

@@ -435,7 +435,7 @@ public static RunQueryRequest query(StructuredQuery... query) {
   public static RunQueryRequest query(
       @Nullable ByteString transactionId, StructuredQuery... query) {
     RunQueryRequest.Builder request = RunQueryRequest.newBuilder();
-    request.setParent(LocalFirestoreHelper.DATABASE_NAME);
+    request.setParent(LocalFirestoreHelper.DATABASE_NAME + "/documents");
     StructuredQuery.Builder structuredQuery = request.getStructuredQueryBuilder();
     structuredQuery.addFrom(
         StructuredQuery.CollectionSelector.newBuilder().setCollectionId("coll"));