(Optionally?) provide trace IDs in all requests, even if open tracing is disabled #11090

This issue has been migrated from #11090.

On #10854 I lamented that it was a bit painful to track down logs for a particular request in a sytest run. In my ideal scenario, each response would have a unique string which identifies the request, so we can grep through logs for that.

One option was to always generate the trace id reported with opentracing, even if opentracing was disabled. I like this approach, and it might make it easier to track future problems reported by clients. See matrix-org/synapse#10854 (comment) and matrix-org/synapse#10854 (comment)

Should probably convince ourselves that there aren't any privacy ramifications in doing this.