Prometheus plugin for c-lightning

This plugin exposes some key metrics from c-lightning in the prometheus format so it can be scraped, plotted and alerts can be created on it. The plugin adds the following command line arguments:

• prometheus-listen: the IP address and port to bind the HTTP server to (default: 127.0.0.1:9750)

Exposed variables include:

• node: ID, version, ...
• peers: whether they are connected, and how many channels are currently open
• channels: fund allocations, spendable funds, and how many unresolved HTLCs are currently attached to the channel
• funds: satoshis in on-chain outputs, satoshis allocated to channels and total sum (may be inaccurate during channel resolution).