[Tracing]: Start root span on Proxy

[refs]

We use an HTTP propagator to go beyond our process boundary. Essentially the span gets serialized in the request context and travels along with the HTTP request. You can test this changes by starting the ocis set of services:

ocis server

stop the proxy: ocis kill proxy
start a new proxy process with the changes from this PR:

# on the root of this repository, on this PR branch
go run cmd/ocis-proxy/main.go --log-level debug server --tracing-enabled=true

Start jaeger default container:

docker run -d --name jaeger \
  -e COLLECTOR_ZIPKIN_HTTP_PORT=9411 \
  -p 5775:5775/udp \
  -p 6831:6831/udp \
  -p 6832:6832/udp \
  -p 5778:5778 \
  -p 16686:16686 \
  -p 14268:14268 \
  -p 14250:14250 \
  -p 9411:9411 \
  jaegertracing/all-in-one:1.18

on the browser: https://localhost:9200, sign in, do some actions.
go to: http://localhost:16686
on the far left, the Service dropdown select proxy
see some traces.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[ownclouders]

Here is an overview of what got changed by this pull request:

Complexity increasing per file
==============================
- pkg/proxy/proxy.go  1
         

See the complete overview on Codacy