Clarify notebook/jupyter_server usage

Description

The code is not a full valid jupyter_server extension as it is based on notebook handler:

jupyter-resource-usage/jupyter_resource_usage/api.py

Line 5 in 77ef234

from notebook.base.handlers import IPythonHandler

Dependency needs update:

jupyter-resource-usage/setup.py

Line 78 in 77ef234

install_requires=["notebook>=5.6.0", "prometheus_client", "psutil>=5.6.0"],

The backport entrypoint is missing:

_load_jupyter_server_extension = load_jupyter_server_extension

Expected behavior

The extension is fully using jupyter_server.

Context

The mix-up was seen following that issue: jupyter-server/jupyter_server#488

After changing the parameter to c.Spawner.cmd = ["jupyter-labhub"], the warning is emitted only for this handler:

python3.7/site-packages/notebook/base/handlers.py:131: RuntimeWarning: Expected to see HubAuthenticatedHandler in <class 'jupyter_resource_usage.api.ApiHandler'>.mro()

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Thanks @fcollonval for the report.

This might also explain some of the issues folks have been having in #91.

Please don't hesitate to send a PR over if you have something to make this better (even as a draft). Thanks!