prom: Handle channel attribute name changes and fix msat conversion

[bleetube]

This is my own swing at #451

I noticed that to_satoshi() was a method defined in pyln-client for the Millisatoshi type, so I made sure to cast values as that type so the conversion works as intended.

This closes #450 which was due to changes in various attribute names for the v23.05 release of CLN.

[cdecker]

Thanks for the PR, I fixed up #451 and we should be good for now, but in case you want to test things locally you can try out the following command:

python3 .ci/test.py prometheus

This will limit the CI runner to the prometheus directory. For each plugin in the CI run the runner will:

1. Determine whether it is a Python/requirements.txt or a Python/Poetry plugin (skipping if it is neither for the moment, but support for non-python plugins is definitely planned).
2. Create a virtualenv to install the dependencies in and to run the tests in
3. Install additional packages we determine to be useful (pytest, pytest-timeout, ...)
4. Run the pytest tests in the virtualenv

We use virtualenvs extensively in CI in the hopes of creating a reproducible environment that devs can also run locally.