How to propogate trace headers

**What content needs to be created or modified?**
Quote from the doc:
> There are no helper methods exposed in Dapr SDKs to propagate and retrieve trace context. You need to use HTTP/gRPC clients to propagate and retrieve trace headers through HTTP headers and gRPC metadata.

There are no examples or info on how to do this

**Describe the solution you'd like**
An example of how to propagate the trace context to service c