Fix prometheus and jeager imports in metrics/export.go

[nijynot]

Building localnet gives e.g. the error

package go.opencensus.io/exporter/prometheus: cannot find package "go.opencensus.io/exporter/prometheus" in any of:
	/home/user/go/src/go.opencensus.io/exporter/prometheus (from $GOROOT)
	/home/user/code/go/src/go.opencensus.io/exporter/prometheus (from $GOPATH)

and is because the packages have been moved moved to contrib.*.

Related comment: census-instrumentation/opencensus-go#1118 (comment)

[travisperson]

I think we need to do this update regardless, but I wasn't able to reproduce this.

My repro attempt was:

go-filecoin $ go clean -modcache
go-filecoin $ go run ./build deps; go run ./build build
localnet $ env GO111MODULE=on go build -o localnet main.go

If you are trying to build, you may try the above to see if it unblock you till we merge this change.

[travisperson]

To accept these changes we just need to update the go.mod + go.sum files. You can do this pretty easily by enabling gomod and running go get

env GO111MODULE=on go get contrib.go.opencensus.io/exporter/prometheus
env GO111MODULE=on go get contrib.go.opencensus.io/exporter/jaeger

Normally you can do this with env GO111MODULE=on go mod tidy, but our go.mod hygiene is a little lacking and it will introduce quite a few changes not related to this PR. So for this PR we can just explicitly update these two.

[nijynot]

Updated the go.mod and go.sum files accordingly so that they include contrib.* prometheus and jaeger.

[frrist]

Thank you for the contribution @nijynot!