prometheus: Change default port to 9750 as per port-allocation

[cdecker]

We registered port 9750 the [port allocation] list for the prometheus project,
so we should use that port :-)

The addition to the list of known exporters is still pending prometheus/docs#1737

Suggested-by: Martin Milata <@mmilata>
Fixes #122
[port allocation]: https://github.com/prometheus/prometheus/wiki/Default-port-allocations

[darosior]

Nice, brian-brazil left some good recommendations on your PR:

Some suggestions to improve your exporter:
_total is a suffix for counters, it shouldn't be used with gauges. Some metrics like lightning_channel_in_msatoshi_offered also sound like they might be counters rather than gauges.
Units are inconsistent, sometimes missing and conflicting with the help text. You should consistently use satoshis, and also include satoshi as the unit in all relevant metric names for clarity.
id is a bit of a generic label, peer or peer_id would be clearer.

I'll open an issue as they would be a great fit for a follow-up

[darosior]

utACK