docs(rfd): Draft: Agent Telemetry Export

[codefromthecrypt]

Define how agents export telemetry (logs, metrics, traces) to clients without tunneling over the ACP transport.

• Clients run a local OTLP receiver and inject OTEL_EXPORTER_* env vars when launching agents
• Keeps telemetry out-of-band to avoid head-of-line blocking on stdio
• Agents use standard OpenTelemetry SDKs with auto-configuration
• No credential sharing: client proxies telemetry to backends with its own credentials
• Complements meta-propagation RFD (trace context correlation)

Implementation proposes a new docs/protocol/observability.mdx covering both client and agent author guidance.

extracted from #295 (comment)