mem-ruby: detailed transaction latency profiling

mem-ruby: detailed transaction latency profiling

Add incomingTransactionStart/End and outgoingTransactionStart/End
functions that can be called from the protocol to profile events
that initiate a transaction locally (e.g. an incoming request) and
remotely (e.g. outgoing requests). The generated stats will include
histograms of the latency for completing each type of transaction.

This assumes assumes the protocol uses different trigger events for
initiating incoming and outgoing transactions.

Change-Id: Ib528641b9676c68907b5989b6a09bfe91373f9c9
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31421
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: John Alsop <johnathan.alsop@amd.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>