metric: document how to use the Registry and export metrics out

[odeke-em]

Please answer these questions before submitting a bug report.

What version of OpenCensus are you using?

Irrelevant

What version of Go are you using?

Irrelevant

What did you do?

I am trying to use this "go.opencensus.io/metric" package to get a gauge for up metrics from the OpenTelemetry collector's prometheus receiver per open-telemetry/opentelemetry-collector#2918, but there are no examples. The only present example is incomplete in https://pkg.go.dev/go.opencensus.io/metric#example-Registry.AddInt64Gauge

The package is challenging to use. For context I used to work on these libraries and wrote and lead a huge chunk of OpenCensus / OpenTelemetry exporters and libraries, but I can't use this package and have digging for very many hours.

What did you expect to see?

A straightforward way to use these libraries

What did you see instead?

Impossible to use them to export out metrics, and work is stalled. I think for now I am going to keep using the straightforward go.opencensus.io/stats/view code given that the OpenTelemetry collector itself uses those.

Kindly cc-ing @bogdandrutu @rakyll @alolita @Aneurysm9

[punya]

Thanks for the feedback @odeke-em. Do you have any suggestions for how to proceed? We'll start looking into this as well.