Proxy filter to propagate Openzipkin b3 headers

[samutamm]

Is your feature request related to a problem? Please describe.
Easegress supports sending tracing spans per request to Zipkin tracing system. These spans can help to troubleshoot latency problems in the downstream back-end systems that Easegress pipelines connect to.

Though sometimes the span information comes in incoming requests in b3-propagation headers and it should be passed along to the downstream back-ends. Currently Easegress does not propagate these headers.

Describe the solution you'd like
When b3 headers are present, propagate them in the Proxy filter to the downstream back-end servers, as desribed in https://github.com/openzipkin/b3-propagation . If b3 headers are not present, generate a new span and inject to it to the header.

Describe alternatives you've considered

Additional context

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