Fix Grpc streaming handling of Authorization header

[mdisibio]

What this PR does:
The gRPC streaming endpoints are unintentionally dropping auth-related headers when issuing jobs to the queriers, so streaming + auth fails. We need to extract them from the gRPC metadata and include them in the job. The fix is here, but it is for downstream Grafana Enterprise Traces

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[joe-elliott]

Should we put this in prepareRequestForQueriers? it's what i generally think of as the place where all the headers are lined up for the queriers:

https://github.com/grafana/tempo/blob/main/modules/frontend/frontend.go#L311

[mdisibio]

Should we put this in prepareRequestForQueriers?

It should be possible since the request parameter's context should still have a copy of the gRPC metadata. However it is called for every job, so it would be less efficient, and I do like that the current location (grpc handlers) limits the scope.

[joe-elliott]

Yup, good call. I do wish there was a way we could centralize the code to make it harder to mess up the next time we add a streaming endpoint.

But I think that about a lot of the boilerplate code between the streaming and http handlers.