frontend: no tracing propagation when downstream_url has been specified

Hi! There are cases when query-frontend is used along with other services like Thanos. The problem with this use-case is in loosing span context in Jaeger.
It happens due to lack of trace initialization in this inline-function: https://github.com/cortexproject/cortex/blob/master/pkg/querier/frontend/frontend.go#L113