Expose /metrics/

[cemeng]

This commit includes prometheus client for node.js (prom-client) and
adds /metrics/ endpoint that a Prometheus server can scrape.

This would address #551

Few notes:

• Trailing slash is needed somehow, /metrics will lead to 404. I am not that familiar with express, if anyone can help that'd be great, ideally we want both /metrics and /metrics/ to show metrics.
• The default metrics exposed might not be enough to be useful, for example: it doesn't have http request time etc - these need to be added as custom metrics.