track latency from request -> showing terminal completions, indicate first shown for shell type and for window

[meganrogge]

part of #251994

addressed

[Copilot]

Pull Request Overview

This PR adds telemetry to measure latency from a terminal completion request to when suggestions are shown and flags the first time completions are displayed per window and per shell type.

• Introduces logCompletionLatency in TerminalSuggestTelemetry to send latency and first-shown flags.
• Updates SuggestAddon to timestamp requests, compute/display latency, and track first-shown state.
• Adds a new FirstShown storage key in terminalStorageKeys.ts.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/vs/workbench/contrib/terminalContrib/suggest/browser/terminalSuggestTelemetry.ts	Added logCompletionLatency method with its telemetry schema.
src/vs/workbench/contrib/terminalContrib/suggest/browser/terminalSuggestAddon.ts	Implemented request timestamping, first-shown determination, and telemetry logging.
src/vs/workbench/contrib/terminal/common/terminalStorageKeys.ts	Added FirstShown enum entry for storing first-shown state.

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/terminalContrib/suggest/browser/terminalSuggestTelemetry.ts:58

• The new logCompletionLatency method adds critical telemetry logic but lacks accompanying unit or integration tests. Please add tests to verify the correct payload is sent under various conditions (e.g., first vs. subsequent invocations).

	logCompletionLatency(sessionId: string, latency: number, firstShownFor: { window: boolean; shell: boolean }): void {

[meganrogge]

Just realized a set for the shell types might be better