Report when buffered events are flushed to a late listener

[dmitrivMS]

Ref #234393

This is to investigate what events take actually use buffer today.

[Copilot]

Pull request overview

This PR adds diagnostic telemetry to identify when ProxyChannel.fromService IPC event buffering is actually used (i.e., buffered events get flushed to a late-attaching listener). It extends Event.buffer with an optional callback to report buffered event counts when the first listener attaches, and wires that into ProxyChannel plus key processes (electron main, shared process, agent host).

Changes:

• Extend Event.buffer to optionally report the buffered event count when the first listener attaches.
• Add a ProxyChannel process-wide buffer-usage reporter hook and emit telemetry (ipc.proxyChannelBufferUsed) when buffered events are delivered.
• Install this diagnostic telemetry reporter in electron-main, shared process, and agent host entry points.

Show a summary per file

File	Description
src/vs/platform/telemetry/common/telemetryUtils.ts	Adds installer that hooks ProxyChannel buffering and logs telemetry when buffering is used.
src/vs/platform/agentHost/node/agentHostMain.ts	Installs the proxy-channel buffer usage telemetry in the agent host process.
src/vs/code/electron-utility/sharedProcess/sharedProcessMain.ts	Installs the proxy-channel buffer usage telemetry in the shared process.
src/vs/code/electron-main/app.ts	Installs the proxy-channel buffer usage telemetry in the electron main process IPC channel init.
src/vs/base/test/common/event.test.ts	Adds unit tests validating the new “report buffered count on first listener” behavior.
src/vs/base/parts/ipc/common/ipc.ts	Introduces a global ProxyChannel.setBufferUsageReporter and wires it into buffered IPC events.
src/vs/base/common/event.ts	Extends Event.buffer signature + implementation to optionally report buffered count when first listener attaches.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 2
• Review effort level: Low