feat(spanner): add support for external hosts

Author: sagnghos

google-cloud-spanner/src/main/java/com/google/cloud/spanner/DatabaseId.java

@@ -94,6 +94,11 @@ public static DatabaseId of(String project, String instance, String database) {
     return new DatabaseId(new InstanceId(project, instance), database);
   }
 
+  /** Creates a {@code DatabaseId} given instance and database IDs. */
+  public static DatabaseId of(String instance, String database) {
+    return new DatabaseId(new InstanceId("default", instance), database);
+  }
+
   /** Creates a {@code DatabaseId} given the instance identity and database id. */
   public static DatabaseId of(InstanceId instanceId, String database) {
     return new DatabaseId(instanceId, database);

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionClient.java

@@ -38,6 +38,8 @@ static class SessionId {
     private static final PathTemplate NAME_TEMPLATE =
         PathTemplate.create(
             "projects/{project}/instances/{instance}/databases/{database}/sessions/{session}");
+    private static final PathTemplate EXTERNAL_HOST_NAME_TEMPLATE =
+        PathTemplate.create("instances/{instance}/databases/{database}/sessions/{session}");
     private final DatabaseId db;
     private final String name;
 
@@ -49,10 +51,16 @@ private SessionId(DatabaseId db, String name) {
     static SessionId of(String name) {
       Preconditions.checkNotNull(name);
       Map<String, String> parts = NAME_TEMPLATE.match(name);
+      if (parts == null) {
+        parts = EXTERNAL_HOST_NAME_TEMPLATE.match(name);
+      }
       Preconditions.checkArgument(
           parts != null, "Name should conform to pattern %s: %s", NAME_TEMPLATE, name);
       return of(
-          parts.get("project"), parts.get("instance"), parts.get("database"), parts.get("session"));
+          parts.containsKey("project") ? parts.get("project") : "default",
+          parts.get("instance"),
+          parts.get("database"),
+          parts.get("session"));
     }
 
     /** Creates a {@code SessionId} given project, instance, database and session IDs. */