Agent label reporting incorrect version of exporter

[chmking]

Please answer these questions before submitting a bug report.

What version of the Exporter are you using?

0.12.4

What version of OpenCensus are you using?

0.22.0

What version of Go are you using?

1.12.4

What did you expect to see?

g.co/agent: opencensus-go 0.22.0; stackdriver-exporter 0.12.4

What did you see instead?

g.co/agent: opencensus-go 0.22.0; stackdriver-exporter 0.10.0

It looks like this is hardcoded to 0.10.0 in stats.go.

const (
	maxTimeSeriesPerUpload    = 200
	opencensusTaskKey         = "opencensus_task"
	opencensusTaskDescription = "Opencensus task identifier"
	defaultDisplayNamePrefix  = "OpenCensus"
	version                   = "0.10.0"
)

var userAgent = fmt.Sprintf("opencensus-go %s; stackdriver-exporter %s", opencensus.Version(), version)