Use opentracing_grpc_propagate_context when necessary

[andremarianiello]

What this PR does / why we need it:
Opentracing header propagation via gRPC

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

fixes #3440

Special notes for your reviewer:
Second half of #3465

[diazjf]

Looks good, still needs some unit tests here https://github.com/kubernetes/ingress-nginx/blob/master/internal/ingress/controller/template/template_test.go

[andremarianiello]

@diazjf Unit test added

[aledbf]

@andremarianiello please run ./hack/verify-gofmt.sh to see the format error

[andremarianiello]

@aledbf Sorry, new dev machine, didn't set up autoformat. Pushed the formatting fixes.

[andremarianiello]

CI is still failing, like its using the code before my fixes, but I don't know why that is happening. Still investigating.

[andremarianiello]

I was using gofmt in Go 1.10, not 1.11. Fixing now

[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, andremarianiello

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[aledbf]

@andremarianiello thanks!

[andremarianiello]

@aledbf Thank you! Is there a planned release date for 0.22.0?

[aledbf]

@andremarianiello we release every three or four weeks