servoshell: allow event tracing to be configured with RUST_LOG

[delan]

To make debugging easier, servoshell logs events from winit, from servo, and to servo, but the output of these logs can be so noisy as to be unusable, unless we reconfigure which event types are logged at compile time.

We use log and env_logger, which allows runtime configuration with RUST_LOG, but by default, while you can filter logs by module path or filter them to only messages that match a regex, you can’t filter them to exclude messages by regex. Custom targets allow us to key our logs on an arbitrary string (as long as it doesn’t contain ,=/).

This patch sets custom targets on servoshell’s event tracing logs, allowing them to be filtered by RUST_LOG. The targets are of the form “servoshell<[source]@[event]” or “servoshell>[destination]@[event]”. For example:

• to trace only events from servo:
  RUST_LOG='servoshell<=off,servoshell>=off,servoshell<servo@'
• to trace all events except for AxisMotion events:
  RUST_LOG='servoshell<,servoshell>,servoshell<winit@WindowEvent(AxisMotion)=off'
• to trace only winit window moved events:
  RUST_LOG='servoshell<=off,servoshell>=off,servoshell<winit@WindowEvent(Moved)'

You can get a very usable setup that traces most unusual events with:

RUST_LOG='warn,servoshell<,servoshell>,servoshell<winit@DeviceEvent=off,servoshell<winit@MainEventsCleared=off,servoshell<winit@NewEvents(WaitCancelled)=off,servoshell<winit@RedrawEventsCleared=off,servoshell<winit@RedrawRequested=off,servoshell<winit@UserEvent(WakerEvent)=off,servoshell<winit@WindowEvent(CursorMoved)=off,servoshell<winit@WindowEvent(AxisMotion)=off,servoshell<servo@EventDelivered=off,servoshell<servo@ReadyToPresent=off,servoshell>servo@Idle=off,servoshell>servo@MouseWindowMoveEventClass=off'

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #___ (GitHub issue number if applicable)

• There are tests for these changes OR
• These changes do not require tests because they only affect debug logging

[mrobinson]

Seems pretty reasonable. My only doubts are about the names (typical, I know 😅).