* drop stack driver as a tracing backend
* drop stackdriver as a metrics backend
* update deps - dropping stackdriver
* fix linting issues
* drop further references to stack driver
* drop serving & eventing metric key constants
these have been moved to their respective repos
see: https://github.com/knative/pkg/issues/608
* move source metrickeys to pkg/source
We're going to move the StatsReporter downstream eventually
related:
https://github.com/knative/eventing/pull/5587https://github.com/knative/eventing/pull/5586
* fix linter
Dave Protasowski
Matt Moore