Add Prometheus /metrics endpoint to allow for easy monitoring

[frittentheke]

I am wondering if a /metrics endpoint for Prometheus could be added to Oxidized to allow to integrate it easily into modern monitoring stacks. There are more tools implementing the scraping Prometheus / OpenMetrics format.

I even found an exporter for the Oxidized HTTP API at https://github.com/momorientes/oxidized_exporter which implements parsing the /nodes.json to then provide metrics. But I'd rather avoid another piece of software / an exporter when this could easily be done within Oxidized.

There also is a client library for Ruby available from Prometheus, see https://github.com/prometheus/client_ruby. There is also a middleware to provide an exporter endpoint https://github.com/prometheus/client_ruby#rack-middleware With all the metrics https://github.com/prometheus/client_ruby#metrics available monitoring could not only cover "success" or "errors", but also performance histograms for the collection of configs.

[robertcheramy]

Prometheus is an open-source systems monitoring and alerting toolkit.

[frittentheke]

@robertcheramy could you revive this one?