Instrument Rack middleware

[thijsc]

If there are Rack middlewares running before your requests we should instrument these so you can see how they influence performance.

Requests

• https://github.com/appsignal/appsignal-server/issues/2514

[tombruijn]

If at all possible, find a way to track errors inside ActionDispatch::ShowExceptions and ActionDispatch::DebugExceptions, even if they're kind of private errors. Namely ActionController::RoutingError for routing errors and non existing routes

As suggested here:
https://app.intercom.io/a/apps/yzor8gyw/inbox/conversation/6039076300?nsrc=slack

[tombruijn]

Review how we would want to do this in combination with #329
Timeboxed research about methods we can do this with one or more middlewares and potential rack monkey patches.

[tombruijn]

https://app.intercom.io/a/apps/yzor8gyw/respond/inbox/540709/conversations/11368616241

[tombruijn]

• start only one transaction per request. If another transaction is active, don't start a new one.
• HAve the middleware complete their own transaction? Rather than Appsignal::Transaction.complete_current!

[tombruijn]

Related issue: #329

[jeffkreeftmeijer]

https://app.intercom.io/a/apps/yzor8gyw/inbox/inbox/unassigned/conversations/16660366147

[tombruijn]

Closing in favor of #329