[feature] Proxy headers more options or proxy all

[telemmaite]

Is your feature request related to a problem? Please describe.
We are using distributed tracing for example https://opentracing.io/, https://zipkin.io/ (https://zipkin.io/pages/architecture.html, https://github.com/openzipkin/b3-propagation)

We have the option to trace request from from frontend > through backend > back to frontend.
This means a few trace headers are added, as quoted:

For example, when an operation is being traced and it needs to make an outgoing http request, a few headers are added to propagate IDs. Headers are not used to send details such as the operation name.

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Describe the solution you'd like

1. Would it be possible to allow proxying all headers, not just this limited list.
   

   centrifugo/internal/proxy/http.go

   Line 74 in 02d489a

   var proxyHeaders = []string{

2. Alternatively allow configuration of list of extra headers, like X-B3-TraceId, X-B3-SpanId, X-B3-Sampled etc...

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Describe alternatives you've considered
Missing part of our tracing log for all proxied requests from centrifugo, if the headers are not passed our backend will assume it is the first entry point and we will add the headers on the trip back.

[FZambia]

@telemmaite yeah, this is reasonable to have. We should not proxy all headers as those can contain a lot of headers that do not have much sense like Connection: Upgrade, Pragma: no-cache etc, but having a white list of extra headers is helpful.

Please look at #335 - is it ok?

[telemmaite]

@FZambia looks great and thank you very much for your effort on this project!

[FZambia]

@telemmaite hi, released new version with new proxy features - v2.3.1

[telemmaite]

@FZambia thank you! Already using the data object with the proxy authentication.