Add OpenCensus reader Bridge, and Prometheus Bridge option

[dashpole]

This is a way to unblock the collectors' use of the opencensus bridge + prometheus exporter. It adds a WithBridge option to the prometheus exporter. It only works for prometheus, and not other pull exporters.

Example usage:

// Create the opencensus bridge metric reader, and pass it to the prometheus exporter as its "source" of metrics.
collector := prometheus.NewExporter(
    prometheus.WithBridge(opencensus.NewPrometheusBridge()),
)
// use the prometheus exporter as usual:
registry := prometheus.NewRegistry()
err := registry.Register(collector)
if err != nil {
	fmt.Printf("error registering collector: %v", err)
	return
}

log.Printf("serving metrics at localhost:2222/metrics")
http.Handle("/metrics", promhttp.HandlerFor(registry, promhttp.HandlerOpts{}))
err = http.ListenAndServe(":2222", nil)
if err != nil {
	fmt.Printf("error serving http: %v", err)
	return
}

This does not align with what is currently being discussed in the specification PR, but will solve the problem for the collector while we work on that.

[codecov]

Codecov Report

Merging #3207 (3777f81) into main (aca054b) will increase coverage by 0.3%.
The diff coverage is 88.8%.

❗ Current head 3777f81 differs from pull request most recent head 91e170f. Consider uploading reports for the commit 91e170f to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3207     +/-   ##
=======================================
+ Coverage   77.3%   77.6%   +0.3%     
=======================================
  Files        159     159             
  Lines      11184   11222     +38     
=======================================
+ Hits        8651    8719     +68     
+ Misses      2335    2303     -32     
- Partials     198     200      +2     

Impacted Files	Coverage Δ
exporters/prometheus/exporter.go	88.1% <86.3%> (+9.6%)	⬆️
bridge/opencensus/metric.go	91.8% <91.3%> (+91.8%)	⬆️
sdk/trace/batch_span_processor.go	81.1% <0.0%> (+0.8%)	⬆️

[paivagustavo]

This looks good! Only one small comment.

[dashpole]

Notes from the SIG meeting:

• The spec PR seems so close! If we can get agreement soon-ish, it would be better to align with the spec, even if it isn't released.
  • @jmacd will try and work out some of the recent disagreements that cropped up (thanks!).
• If the spec PR is still up in the air by 9/26, I'll move forward with something similar to this PR

[dashpole]

I implemented the idea I had at the end of the sig meeting, which is to have prometheus define an interface without the rest of the Reader functions. It makes it much less bad IMO.

It is now prometheus-specific, but that could be changed if we put the Bridge interface somewhere else.

[dashpole]

Note that this provides a ScopeMetrics, rather than a ResourceMetrics. This ensures we only get the resource from the SDK.

[jmacd]

Looks good.

[dashpole]

This was a bug I found while adding unit tests. This allows the bridge to have partial success sending metrics, and keeps the behavior of the previous OC bridge implementation.

[dashpole]

This is ready for review now.

[paivagustavo]

Thanks @dashpole!

As part of planning the migration of the otel collector instrumentation from OpenCensus to Otel Go, this would help us being able to have an incremental and hopefully smoother migration of the existing instrumentation in the collector core and contrib project.

Without the bridge our option would be to migrate the whole project (and contrib) at once, or migrate it incrementally to otel go losing some metrics in the first moment until everything gets migrated.