Handle distributed context headers in custom propagator

[greenEkatherine]

Resolves #1497

Added ReverseProxyPropagator that handles headers removal. Default inner implementation removes distributed context headers. It can be changed by overriding Propagator property in ForwarderHttpClientFactory which is used to set up handler.ActivityHeadersPropagator.

[MihaZupan]

We should update the direct-forwarding docs to include changing this property on the SocketsHttpHandler as well:
https://github.com/microsoft/reverse-proxy/blob/96ed54fd0286638838edb1543333a00dbb3c595b/docs/docfx/articles/direct-forwarding.md?plain=1#L51-L57

As for the breaking-change: anyone using direct forwarding will have to set the ActivityHeadersPropagator property to match existing behavior.

[greenEkatherine]

We should update the direct-forwarding docs to include changing this property on the SocketsHttpHandler as well: https://github.com/microsoft/reverse-proxy/blob/96ed54fd0286638838edb1543333a00dbb3c595b/docs/docfx/articles/direct-forwarding.md?plain=1#L51-L57

I noticed that example there doesn't match with Direct.Sample and not only with this new property. Should I update Direct.Sample as well?

[MihaZupan]

Yes please.

We should also consider updating https://microsoft.github.io/reverse-proxy/articles/header-guidelines.html#traceparent-request-id-tracestate-baggage-correlation-context to mention you can turn header removal off by calling

.ConfigureHttpHandler((_, handler) => handler.Propagator = null)

(might not be the exact syntax) in Startup

[MihaZupan]

https://github.com/microsoft/reverse-proxy/blob/96ed54fd0286638838edb1543333a00dbb3c595b/samples/ReverseProxy.Direct.Sample/Startup.cs#L36-L42

should include the ActivityHeadersPropagator.

[greenEkatherine]

https://github.com/microsoft/reverse-proxy/blob/96ed54fd0286638838edb1543333a00dbb3c595b/samples/ReverseProxy.Direct.Sample/Startup.cs#L36-L42

should include the ActivityHeadersPropagator.

Oh, forgot this one, adding...