1731279 - Investigate starting RLB shutdown a little earlier

Status: NEW

(Toolkit :: Telemetry, task, P5)

Description

[Chris H-C :chutten]

From dthayer's comment it appears as though we can be more pleasant during shutdown by:

1. Starting our shutdown earlier during AppShutdownTelemetry
   • This should be an okay thing to do for our consumers who are already used to this being the final point in time that Telemetry can be recorded
2. Blocking on our shutdown tasks completing during XPCOMShutdown

This might be a little tricky given how the Rust Glean SDK currently exposes its shutdown, but we can change that (maybe leave shutdown()'s timing as-is (only returns when shutdown is complete), but introduce a begin_shutdown() which kicks things off and then returns immediately?).