Add Redis backend for Perfetto traces

[rmosolgo]

Extracted from #5244 -- I'm trying to figure out where the failing snapshot test came from...

[github-actions]

instance method end_execute_multiplex missing tests for lines 196, 197, 198, 199, 200, 196, 196 (coverage: 0.375)

[rmosolgo]

Ok, I found the error. Fiber[:graphql_flow_stack] was being left as an empty array from one run to the next. Then later runs would use that top-level array instead of a per-fiber array created later on. I think there is actually more jankiness here, but unsetting that before each run will contain the jankiness to one query.

I think better behavior for the flow stack would be:

• When pushing a flow packet, push it to this fiber's stack (not a parent fiber)
• When reading a flow packet, read it from this fiber's stack, or a parent fiber (the current fiber may have been created to finish work started on another fiber)

But I'm not going to implement that now.