Consider adding a router configuration option to reject requests without tracing data

[klingerf]

This suggestion came up in conversation with @harryzoomed. Right now when linkerd receives an http request that does not include a trace id, it initializes a new trace for the request:

https://github.com/BuoyantIO/linkerd/blob/master/linkerd/protocol/http/src/main/scala/com/twitter/finagle/buoyant/linkerd/HttpTraceInitializer.scala#L28

That is the correct behavior for when a request first enters the system. For additional downstream requests, however, we expect requests to already have their trace id set, and not having a trace id might be an indication that the request was mishandled by an upstream service. In that case, it could be helpful to expose a router configuration option, such that we could configure an internal router to reject requests that are missing their tracing data.

[olix0r]

At the least, I think we could expose a counter that tracks the number of newly-minted trace ids.