Prometheus metrics

[mhumpula]

Description

This is a basic prometheus metrics exporter of instances metrics. The first commit adds a prometheus depdency libraries (would be nice if #1104 were merged first). The second commit exports instance status information so it's possible to create alerting rules in prometheus to alert on undesired states like not replicating slave.

I think it would be possible to add existing orchestrator metrics from go-metrics module.

Related issue: #355

[shlomi-noach]

Thank you for this!
Question: are 1396 files really necessary? That's three times more than the current number of dependency files. I'm a bit afraid of uncontrolled dependency list. at some time we will switch to go modules, where I feel control is even weaker.

[mhumpula]

Thank you for this!
Question: are 1396 files really necessary? That's three times more than the current number of dependency files. I'm a bit afraid of uncontrolled dependency list. at some time we will switch to go modules, where I feel control is even weaker.

I don't know... technically speaking I'm not a golang programmer, so I have no idea, what is the proper way to actually do this. I have copied one module at time to make the code compile successfully, so I think that this is the minimal set of modules. But it should be possible to edit the source code of prometheus module, so it doesn't need some dependencies (like protobuf). Not sure if it should be part of the vendor after that.