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

This issue has been migrated from [#11090](https://github.com/matrix-org/synapse/issues/11090).

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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 https://github.com/matrix-org/synapse/pull/10854#issuecomment-925749359 and https://github.com/matrix-org/synapse/pull/10854#issuecomment-933357047

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