Websocket tracing spans

[bboreham]

Since the http call lasts forever, the spans for it don't appear.
Instead, start a new root span every time round the loop (which is typically every 5 seconds).

For the multitenant case it's useful to have a tag with the userid.

[fbarl]

Code looks good! 👌

Up to you whether you want to address my small comments.

[fbarl]

Thanks for the changes, feel free to merge now!