Track the Jetty request span in Cloud Trace

[meltsufin]

The span of the request while being processed by Jetty should be reported to Cloud Trace by adding a span to the trace ID found in the x-cloud-trace-context HTTP header.
It should be possible to optionally enable/disable this feature.

[meltsufin]

Perhaps use Open Census for this in order to enable tracing for gRPC calls as well: https://github.com/census-instrumentation/opencensus-java/tree/master/exporters/trace_stackdriver#quickstart

[gregw]

Note that for other reasons we are working on adding a new listener to the next jetty 9.4 release that will make it much simpler to track all phases of a request/response life cycle - request line, headers complete, body complete, commit response, response complete.

[meltsufin]

Thanks for the heads of up Greg! It sounds like it would be good place to plug in our tracing.

[gregw]

Note that HttpChannel.Listeners were added in 9.4.8, which should be ideal for this task.