Previously ActionDispatch::ServerTiming would subscribe and unsubscribe
on each request. This could cause issues with the internal stacks of
ActiveSupport::Notifications, particlularly under the previous AS::N
implementation which used thread-local stacks for every subscriber
(the new implementation has mostly mitigated this).

Additionally, the previous ServerTiming implementation did not report
metrics correctly in a multi-threaded environment.

This commit works around both of these issues by using a single global
subscription, which collects events into a per-thread Array.