Add a generic host-validated extension telemetry gRPC API

[huimiu]

Task

azd extensions currently have no governed way to contribute telemetry to azd's own command events. Add a generic, host-controlled gRPC API so an authenticated extension can report a host-validated telemetry value, and register the first field (agent.deploy.mode) so a follow-up extension change can produce it.

This is the core enablement, delivered by #9174. It does not itself emit any value.

Scope

• New gRPC TelemetryService.AddCommandUsageAttribute(key, value); core validates against a host-owned allowlist (key, allowed values, classification, purpose, eligible commands, required capability).
• Command-scoped storage in core so a value attaches to the owning command event (cmd.deploy / cmd.up) only, never leaking onto synthetic child spans or sibling commands.
• Authenticated and capability-gated; invalid input rejected and never echoed into errors or logs.
• Best-effort and backward compatible; never affects deploy; an older host returns Unimplemented.
• Register agent.deploy.mode (code / container / byo_image) as SystemMetadata / FeatureInsight.
• Add a generic azd.fromPackage artifact provenance marker.
• Document the SDK API and the registered field (SDK reference and telemetry schema).

Acceptance criteria

• An authenticated service-target-provider extension can call the API; core validates and records allowed values on the correct command event.
• Unknown keys/values and a missing capability are rejected without recording.
• Telemetry failure never changes deployment success, error, or output.
• No producer required for this task: the field is registered but not yet emitted.

Related

• End goal / producer: Record hosted agent deployment mode telemetry #9230 (record hosted agent deployment mode from the agents extension), lands in a follow-up PR on a released core SDK.
• Open question: allowlist strictness (see feat: add host-validated extension telemetry gRPC service #9174) — keeping the strict host allowlist ties new extension telemetry fields to the core release cadence.