Misleading error message for nil tag key

[mwuertinger]

What version of the Exporter are you using?

v0.12.4

What version of OpenCensus are you using?

v0.22.0

What version of Go are you using?

go version go1.12.7 linux/amd64

What did you do?

I accidentally built and registered a view with a nil tag key:

myView = &view.View{
	Name:        "myview",
	Description: "some description",
	Measure:     myMeasure,
	Aggregation: view.Distribution(1, 2, 3),
	TagKeys:     []tag.Key{nil},
}

What did you expect to see?

I expected to see an informative error message containing at least the affected view.

What did you see instead?

I got the following error message from the Stackdriver exporter which was not helpful in locating the problem:

Request was missing field metricDescriptor.labels[1].key: Label key is missing.