bug: FileSpanExporter crashes with circular JSON serialization error #19228

Bug Description

FileSpanExporter.export() crashes with TypeError: Converting circular structure to JSON when writing OTel spans to a telemetry outfile.

Steps to Reproduce

1. Enable file-based telemetry:

   { "telemetry": { "enabled": true, "target": "local", "otlpEndpoint": "", "outfile": ".gemini/telemetry.log" } }

2. Run GEMINI_DEV_TRACING=true gemini and send any prompt

Error

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'BatchSpanProcessor2'
    |     property '_shutdownOnce' -> object with constructor 'BindOnceFuture'
    --- property '_that' closes the circle

Root Cause

FileExporter.serialize() in packages/core/src/telemetry/file-exporters.ts uses JSON.stringify(data, null, 2) which does not handle circular references. OTel ReadableSpan objects contain circular references through their internal span processor chain.

Suggested Fix

Replace JSON.stringify with the existing safeJsonStringify utility:

+import { safeJsonStringify } from '../utils/safeJsonStringify.js';

 class FileExporter {
   protected serialize(data: unknown): string {
-    return JSON.stringify(data, null, 2) + '\n';
+    return safeJsonStringify(data, 2) + '\n';
   }
 }