Instrument with OpenCensus

[jkohen]

This is for monitoring the sidecar itself, not how we write the Prometheus data.

This is important so we don't rely on Prometheus to monitor the Prometheus sidecar (although that will be an option with OpenCensus).

https://opencensus.io/go/index.html

[jkohen]

@fabxc you already started working on this.

[fabxc]

All parts currently in functionality scope of OC should be migrated to it. Anything further depends on gauge support being added.

[wdhnl]

OpenCensus now supports a view type of LastValue which is like a gauge https://godoc.org/go.opencensus.io/stats/view#LastValue

[jkohen]

Thanks, Bill. We are working with OpenCensus to have a first-class gauge in Go soon, like it was added to Java https://github.com/census-instrumentation/opencensus-java/blob/master/api/src/main/java/io/opencensus/metrics/LongGauge.java

[jkohen]

This is a cleanup. We'll take care of this if necessary, but currently it's not a blocker, so I'm closing the issue.