Add env var to shut down kernel immediately after start for benchmarking

Setting IPYKERNEL_BENCHMARK_STARTUP_SHUTDOWN stops the event loop right
after it starts, so total process wall-clock time measures kernel
startup without needing an actual client to send a shutdown request.

This can be used in conjunction with -X importtime and profile viewer
like tuna to work on speeding up importing and starting a kernel

Author: Carreau

ipykernel/kernelapp.py

@@ -755,6 +755,10 @@ def start(self):
             self.poller.start()
         self.kernel.start()
         self.io_loop = ioloop.IOLoop.current()
+        if os.environ.get("IPYKERNEL_BENCHMARK_STARTUP_SHUTDOWN"):
+            # Shut down immediately after entering the event loop, for
+            # benchmarking kernel startup time end-to-end.
+            self.io_loop.add_callback(self.io_loop.stop)
         if self.trio_loop:
             from ipykernel.trio_runner import TrioRunner