End-to-end overlay testing via ETL file

[planetchili]

It would be very useful to be able to take an ETL file from a user experiencing issues and play it back in realtime to observe the overlay graph output it generates. This would be particularly valuable in situations where the ETL seems to run fine through standalone OPM processing.

Note that telemetry data would not be applicable to such situations. Typically this is not a problem as most issues involve present event tracing.

Requirements

• ability to throttle ETL events, pacing them to approximately the same cadence that they were captured at
• ability to adjust timestamps of events, spoofing them to appear as realtime in the playback system
• ability to feed ETL files to the service (with attendant API/Middleware support)
• ability to trigger playback to coincide with start of UI-side tracking
• ability to switch service into a mode that processes from ETL instead of from realtime trace (potentially on a per-client visibility)
• ability to replace telemetry providers/adapters with null versions
• UI support for above

[planetchili]

Above checklist has been partially split off into other tasks, which this task now depends on as prerequisites (#399 #400). This task should be divided into:

1. Supporting replay of canned/generated ETL+telemetry data through the overlay (interactive)
2. Full end-to-end automated testing of overlay using above data via snapshots and/or data buffer dumps