Add Puma request queue time as a transaction attribute

[dingsdax]

Puma queue-time is a critical latency metric for Ruby/RoR apps.
Sentry instruments requests only after Puma begins processing.
Capture queue-time via headers and attach it as a transaction attribute.

others: scout, judoscale

[linear]

RUBY-130

[dingsdax]

I've created a reproduction for this issue: getsentry/repro#3

You can run it by cloning the repo and following the instructions in the README. The reproduction demonstrates:

1. How queue time is available in request headers (X-Request-Start)
2. That Sentry currently does not capture this queue time in transactions
3. How other Ruby APM tools (Scout APM, Judoscale) successfully capture this metric

The simulation script shows a request with ~500ms of queue time - you can see this calculated from headers in the console output, but it's not present in the Sentry transaction data.