With this change folks will need to call `webhook.RegisterMetrics()` to register the opencensus view with the metrics for the webhook's StatsReporter. This is needed to avoid having `sharedmain` crashloop the activator due to linking multiple on-`init()` views that register metrics named `request_latencies`.
In general, I believe that we should move away from registering these views via `init()` and more towards the broader K8s MetricsProvider pattern.
* Add metrics to webhook package
Add metricstest package for shared helper functions for testing metrics
* Address PR
* Cleanup
* Fix import paths to fix build issues
* Fix import package path for test file
* Remove unnecessary formatting from error message
* Remove helper function only used once
* Add metric name to all error messages, make checkRowTags testing helper function
* Add common histogram bucket generator function to metrics package
* Fix CheckStatsNotReported check
* Reset metrics before each test so the tests are idempotent
* Make CheckStatsNotReported conditional clearer
Matt Moore
Annie Fu