prom: Fix breakage due to msat purge

[cdecker]

Closes #450

[bleetube]

Probably a red herring (ie. something I did wrong) but using this branch with v23.05.1 resulted in:

Exception occurred during processing of request from ('127.0.0.1', 40712)
Traceback (most recent call last):
  File \"/usr/lib/python3.11/socketserver.py\", line 691, in process_request_thread
    self.finish_request(request, client_address)
  File \"/usr/lib/python3.11/socketserver.py\", line 361, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File \"/usr/lib/python3.11/socketserver.py\", line 755, in __init__
    self.handle()
  File \"/usr/lib/python3.11/http/server.py\", line 432, in handle
    self.handle_one_request()
  File \"/usr/lib/python3.11/http/server.py\", line 420, in handle_one_request
    method()
  File \"/home/lightning/.local/lib/python3.11/site-packages/prometheus_client/exposition.py\", line 151, in do_GET
    output = encoder(registry)
             ^^^^^^^^^^^^^^^^^
  File \"/home/lightning/.local/lib/python3.11/site-packages/prometheus_client/exposition.py\", line 89, in generate_latest
    for metric in registry.collect():
  File \"/home/lightning/.local/lib/python3.11/site-packages/prometheus_client/registry.py\", line 75, in collect
    for metric in collector.collect():
  File \"/home/lightning/src/plugins/prometheus/prometheus.py\", line 52, in collect
    [o['amount_msat'].to_satoshi() for o in funds['outputs']]
  File \"/home/lightning/src/plugins/prometheus/prometheus.py\", line 52, in <listcomp>  
    [o['amount_msat'].to_satoshi() for o in funds['outputs']]
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'int' object has no attribute 'to_satoshi'

Guessing I am missing another change that would make o['amount_msat'] into something other than just an integer. If so, please disregard.

[cdecker]

Good catch, int(o['amount_msat']) instead of to_msatoshi should work on both old and new versions

[cdecker]

Ok, finally found the time to fix this one up. Now the tests also create a channel and ensure that listpeers and listpeerchannels are parsed correctly.

[bleetube]

It's been a minute, but I think the error I mentioned above was actually due to the attr name change. I submitted a PR a few weeks ago #459

I still need to figure out how to run these internal tests, but they look pretty useful.