Add a header `X-B3-Flags: 1` to the request to Zipkin server  (#8223)
Add a header `X-B3-Flags: 1` to the request to Zipkin server  (#8223)

Add a header `X-B3-Flags: 1` to the request to Zipkin server. It is a debug flag that ensures that the trace will be retained on the server side.
The decision to sample is made on the client side by setting `--reporting-zipkin-sample-rate`. 
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