Clearcut logging - initial implementation (#1274)

Flag-guarded initial implementation of a clearcut logger to collect telemetry data and send it to Concord for dashboards, etc.

packages/core/src/telemetry/index.ts

@@ -25,15 +25,15 @@ export {
   logApiRequest,
   logApiError,
   logApiResponse,
-  getFinalUsageMetadata,
 } from './loggers.js';
 export {
+  StartSessionEvent,
+  EndSessionEvent,
   UserPromptEvent,
   ToolCallEvent,
   ApiRequestEvent,
   ApiErrorEvent,
   ApiResponseEvent,
-  CliConfigEvent,
   TelemetryEvent,
 } from './types.js';
 export { SpanStatusCode, ValueType } from '@opentelemetry/api';