Add a public method to convert OpenCensus View to MetricDescriptor.

[yegle]

This is useful if you want to use your code as source of truth of
MetricDescritors.

Managing MetricDescriptors outside of the Go code (e.g. in Terraform
configs) is painful, as the changes to MD requires delete/recreate,
after deleting all alert policies associated with the metric. On the
other hand, the externally managed MDs are bound to be out of sync with
what's present in the code.

In the end we decide to just extract/collect views from our go code as
source of truth, and thus we need to have a public method to convert
views to MD.

[yegle]

/cc @rghetia PTAL.

[yegle]

Friendly ping :-)

[dashpole]

Doesn't OpenCensus already manage the creation of metric descriptors for you?

opencensus-go-exporter-stackdriver/stats.go

Line 330 in cde3c10

func (e *statsExporter) createMetricDescriptorFromView(ctx context.Context, v *view.View) error {

Is there something it does/doesn't do that makes you want to manage them yourself?

[yegle]

Hmm I think I workarounded the issue by adding proper dependency in our Terraform config.

It used to be an issue when there's no dependency between the Cloud Run service (where the metric is produced) and the alert/dashboard.

I'll close this. Thank you for taking a look!

[yegle]

Hmm I think we still want to have this feature.

The library does take care of the metric creation, but only happen when at lease one call to stat.Record. If the metric is incremented in a condition statement and rarely reaches, the metric descriptor would never be created, and everything depending on the metric (e.g. monitoring dashboard, alerting policy) would have to wait for a very long time before they can be created.

Given this, we plan to use the new function added in this PR to generate a list of metric descriptors and then we can create them manually.

Would it make sense to merge this PR? Thank you!

[codecov-io]

Codecov Report

Merging #277 (9254941) into master (0fc2674) will decrease coverage by 0.77%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #277      +/-   ##
==========================================
- Coverage   72.72%   71.95%   -0.78%     
==========================================
  Files          17       17              
  Lines        1712     1558     -154     
==========================================
- Hits         1245     1121     -124     
+ Misses        391      363      -28     
+ Partials       76       74       -2     

Impacted Files	Coverage Δ
stackdriver.go	39.32% <0.00%> (+1.23%)	⬆️
propagation/http.go	58.62% <0.00%> (-2.67%)	⬇️
sanitize.go	84.61% <0.00%> (-2.06%)	⬇️
metrics.go	76.56% <0.00%> (-1.99%)	⬇️
trace_proto.go	61.58% <0.00%> (-1.83%)	⬇️
stats.go	76.68% <0.00%> (-1.72%)	⬇️
trace.go	46.15% <0.00%> (-1.64%)	⬇️
monitoredresource/aws/monitored_resources.go	66.66% <0.00%> (-1.52%)	⬇️
metrics_proto.go	86.92% <0.00%> (-1.24%)	⬇️
label.go	100.00% <0.00%> (ø)
... and 8 more

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[yegle]

I appreciate your help!

[yegle]

Would it be possible to cut a new release including this PR? Thank you!

[dashpole]

release has been cut