solarnet: add prometheus on pluto

[ghost]

Scrapes all gateway hosts' /debug/metrics/prometheus and provides a query API and UI.

See it in action: http://pluto.i.ipfs.io/prometheus/graph#[{%22range_input%22%3A%221h%22%2C%22end_input%22%3A%22%22%2C%22step_input%22%3A%22%22%2C%22stacked%22%3A%22%22%2C%22expr%22%3A%22ipfs_p2p_peers_total%22%2C%22tab%22%3A0}]

Prometheus runs its default config, with only the scraping targets added. I left out authentication because I'm not sure it needs to be protected. @jbenet @whyrusleeping wdyt?

[jbenet]

@lgierth this is very nice! i like it a lot.

I left out authentication because I'm not sure it needs to be protected.

People could use this information to do monitoring + targetting based on content. not with these metrics, but as we add more, they could. (could even give people a gateway link, and check when they accessed it). Let's put it behind auth for now, as this is a security + privacy concern. We can make it easy to get access -- i.e. give people in the community access as they request it -- but not have a public endpoint where we dont know what people are using it for.

[jbenet]

and code LGTM! 👍

[ghost]

Merging this as-is since it's the current state of pluto, will add authentication first thing tomorrow.