Export site metrics in Prometheus

[labkode]

Run like this:

red:reva gonzalhu$ cmd/revad/revad -c examples/metrics/metrics.toml

Test like this:

red:reva gonzalhu$ curl http://0.0.0.0:9998/metrics
# HELP revad_opencensus_io_http_server_request_count Count of HTTP requests started
# TYPE revad_opencensus_io_http_server_request_count counter
revad_opencensus_io_http_server_request_count 1
# HELP revad_opencensus_io_http_server_request_count_by_method Server request count by HTTP method
# TYPE revad_opencensus_io_http_server_request_count_by_method counter
revad_opencensus_io_http_server_request_count_by_method{http_method="GET"} 1

You may want to copy this branch into one of yours

[redblom]

@labkode Is it possible to make the site metrics to somehow show up between the current prometheus service metrics (displayed through the exporter). I can only get them displayed if I let the current prometheus service New() function simply return with a promhttp.Handler() and remove/ignore the exporter code. Like so:
return &svc{prefix: conf.Prefix, h: promhttp.Handler()}, nil

[labkode]

@ishank011 can you please take a look a this corner?

[ishank011]

@labkode Will look at it 👍

[ishank011]

@labkode @redblom We can define the metrics in the metrics package as has been done in #740, and then import those in the prometheus HTTP service and register them as separate views as done here, and then initialize the counters. Initializing is important, otherwise the metrics don't show. In the metrics pkg, we can define the methods to update the stats.

And since we're using opencensus metrics in the prometheus service, it'd be better in my opinion if we can use the go.opencensus.io/stats package to store these, like done here.

[redblom]

@ishank011 Thank you for the explanation. I can implement this fine, this works.
@labkode I pushed the new commits.