README: add Prometheus plugin

[USA-RedDragon]

I created a Prometheus metrics plugin at https://github.com/USA-RedDragon/trunk-recorder-prometheus, so I wanted to link it here. If this isn't something you want to link, feel free to close the PR, no hard feelings :)

[robotastic]

This is great!! I am going to go give this a try - it seems like a super easy way to track stats.

[frameshift18]

Trying to get this to work and ran into a couple issues.

1. In trunk-recorder/plugin_manager/plugin_api.h I had to change #include <json.hpp> to #include "../json.hpp" or else it couldn't find it. I know you just changed that so probably a typo?

2. When trying to build the plugin I get the following error and I don't know enough about C to fix it, lol :D. I did remove 'override' and it built but it doesn't seem to work then. At least nothing loads at the :9842 endpoint and Prometheus doesn't find any metrics.

prometheus_plugin.cc:60:7: error: ‘int Prometheus::parse_config(boost::property_tree::ptree&)’ marked ‘override’, but does not override 60 | int parse_config(boost::property_tree::ptree &cfg) override

[USA-RedDragon]

Yeah the plugin was built against 4.2.0, so the new json stuff being passed around in the latest changes on the master branch aren't used yet since the plugin API changed a bit (#856) this morning. Getting my plugin's main based on the master is on my todo tonight. There now exists a 4.2.0 tag on the GHCR image which should run against Trunk Recorder 4.2.0 if you're using Docker

[frameshift18]

Kind of figured. Course I had to update my running TR instance to master and then try this. All good and appreciate the work :)

[robotastic]

Sorry about that - I was going to put in a PR to fix things.