Inject session identifier into environment variable.

There are many use case where users want to know the current notebook
name/path. This help by adding a session identifier (to not really say
this is the current notebook name), and by default make it the full path
to the notebook document that created the session.

This will of course not work if the notebook get renamed, but we can
tackle this later.

See also jupyter/jupyter_client#656,
jupyter/notebook#6180. It will need to be ported
to jupyter_server as well.

This is the mirror commit of
jupyter/notebook#6279 on main notebook.

Author: Carreau

jupyter_server/services/sessions/sessionmanager.py

@@ -272,7 +272,9 @@ async def start_kernel_for_session(self, session_id, path, name, type, kernel_na
         # allow contents manager to specify kernels cwd
         kernel_path = self.contents_manager.get_kernel_path(path=path)
         kernel_id = await self.kernel_manager.start_kernel(
-            path=kernel_path, kernel_name=kernel_name
+            path=kernel_path,
+            kernel_name=kernel_name,
+            env={"JPY_SESSION_NAME": path},
         )
         return kernel_id