Telemetry outfile writes "[Circular]" for shared metric endTime and histogram boundaries #29406

What happened?

With telemetry written to a file (GEMINI_TELEMETRY_TARGET=local plus GEMINI_TELEMETRY_OUTFILE), most metric data points in the file have "endTime": "[Circular]" instead of a timestamp, and histogram bucket boundaries are written as "[Circular]" too.

The cause is safeJsonStringify in packages/core/src/utils/safeJsonStringify.ts. It adds every object it visits to a WeakSet and never removes it, so any object that is referenced a second time is replaced with "[Circular]", even when there is no cycle:

const shared = { x: 1 };
safeJsonStringify({ a: shared, b: shared });
// {"a":{"x":1},"b":"[Circular]"}

The OpenTelemetry SDK reuses one endTime array for all data points in a collection, and one boundaries array per histogram, so every data point after the first loses them. The file exporters started using safeJsonStringify in #19244 to get past the real span processor cycle from #19228.

Other callers have the same problem whenever an object is shared, for example RecordingContentGenerator (--record-responses) and the gen_ai tool definition attributes in LoggingContentGenerator.

Repro on main (09e048f), headless, no account needed:

cat > fake.responses <<'EOF'
{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"text":"Hello there."}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":10,"candidatesTokenCount":3,"totalTokenCount":13}}]}
{"method":"generateContent","response":{"candidates":[{"content":{"parts":[{"text":"{}"}],"role":"model"},"finishReason":"STOP","index":0}]}}
{"method":"countTokens","response":{"totalTokens":10}}
EOF
GEMINI_CLI_TRUST_WORKSPACE=true GEMINI_TELEMETRY_ENABLED=true GEMINI_TELEMETRY_TARGET=local \
  GEMINI_TELEMETRY_OUTFILE=$PWD/telemetry.log GEMINI_API_KEY=fake \
  node packages/cli/dist/index.js -p "say hello" --fake-responses-non-strict $PWD/fake.responses
grep -c '"endTime": \[' telemetry.log              # 3
grep -c '"endTime": "\[Circular\]"' telemetry.log  # 30
grep -c '"boundaries": "\[Circular\]"' telemetry.log  # 17

What did you expect to happen?

Only real cycles should become "[Circular]". In that run those are the 2 _that entries from the span processor. Every endTime and boundaries value should be written out.

Client information

Login information

Not needed for the repro (--fake-responses-non-strict, dummy API key).

Anything else we need to know?

I have a fix ready: track the objects on the current path from the root, not every object seen, so only an object that contains itself is marked. With it, the same run writes all 33 endTime arrays and all histogram boundaries, and the two _that cycles are still marked [Circular]. The existing cycle tests in safeJsonStringify.test.ts and file-exporters.test.ts pass unchanged. I added tests for shared references, and they fail on main. Happy to open the PR if that works for you.