SSE stream of this org's telemetry signals

Server-sent events carrying spans, metrics and events for the caller's organization. Human users only: an agent must not be able to subscribe to org-wide telemetry.

Every message carries an id: field. On reconnect, send the last one back as the Last-Event-ID header to resume. If that id has fallen out of the server's buffer, or came from another replica, the stream opens with an event: gap message and the client must refetch topology rather than assume continuity. A gap is also emitted, without closing the connection, when a subscriber falls behind.