Use singleton prism server by default. (#36228)

* Use singleton prism server by default.

* Modify test since we make prism singleton the default.

Author: shunping

sdks/python/apache_beam/runners/portability/prism_runner.py

@@ -75,9 +75,9 @@ def default_job_server(self, options):
     debug_options = options.view_as(pipeline_options.DebugOptions)
     get_job_server = lambda: job_server.StopOnExitJobServer(
         PrismJobServer(options))
-    if debug_options.lookup_experiment("enable_prism_server_singleton"):
-      return PrismRunner.shared_handle.acquire(get_job_server)
-    return get_job_server()
+    if debug_options.lookup_experiment("disable_prism_server_singleton"):
+      return get_job_server()
+    return PrismRunner.shared_handle.acquire(get_job_server)
 
   def create_job_service_handle(self, job_service, options):
     return portable_runner.JobServiceHandle(

sdks/python/apache_beam/runners/portability/prism_runner_test.py

@@ -461,9 +461,9 @@ class PrismRunnerSingletonTest(unittest.TestCase):
   @parameterized.expand([True, False])
   def test_singleton(self, enable_singleton):
     if enable_singleton:
-      options = DebugOptions(["--experiment=enable_prism_server_singleton"])
+      options = DebugOptions()  # prism singleton is enabled by default
     else:
-      options = DebugOptions()
+      options = DebugOptions(["--experiment=disable_prism_server_singleton"])
 
     runner = prism_runner.PrismRunner()
     with mock.patch(