A1: replayer instrumentation for cross-process join

RequestMetrics gains absolute unix timestamps (t_dispatch_unix,
t_first_token_unix, t_finish_unix), the proxy_request_id, the chosen
endpoint URL, and the trace hash_ids. Replayer sends
X-Request-Id: <session_id>:<turn_id>:<chat_id>:<idx> so proxy
breakdown rows can be joined to metrics by exact key.

Required by Batch 0 (online sequentiality proof) and Batch 1 reuse
decomposition; existing metrics.jsonl couldn't establish either.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_metrics_instrumentation.py

@@ -0,0 +1,75 @@
+"""Tests for A1 replayer instrumentation: new timestamp + join fields."""
+
+from __future__ import annotations
+
+import json
+from dataclasses import asdict
+
+from replayer.metrics import RequestMetrics
+
+
+def _make_metric(**overrides) -> RequestMetrics:
+    base = dict(
+        request_id="sess1:0:42:0",
+        session_id="sess1",
+        turn_id=0,
+        trace_timestamp_s=1.0,
+        input_length=1000,
+        output_length=50,
+        request_type="user",
+        effective_input_length=1000,
+        cached_tokens=512,
+        latency_s=2.5,
+        ttft_s=0.5,
+        tpot_s=0.04,
+    )
+    base.update(overrides)
+    return RequestMetrics(**base)
+
+
+def test_request_metrics_has_new_join_fields():
+    m = _make_metric()
+    fields = asdict(m)
+    for key in (
+        "t_dispatch_unix",
+        "t_first_token_unix",
+        "t_finish_unix",
+        "proxy_request_id",
+        "endpoint_url",
+        "trace_hash_ids",
+    ):
+        assert key in fields, f"missing {key}"
+
+
+def test_request_metrics_defaults_when_unset():
+    m = _make_metric()
+    assert m.t_dispatch_unix is None
+    assert m.t_first_token_unix is None
+    assert m.t_finish_unix is None
+    assert m.proxy_request_id is None
+    assert m.endpoint_url is None
+    assert m.trace_hash_ids == ()
+
+
+def test_request_metrics_round_trips_through_json():
+    m = _make_metric(
+        t_dispatch_unix=1700000000.123,
+        t_first_token_unix=1700000000.5,
+        t_finish_unix=1700000002.0,
+        proxy_request_id="sess1:0:42:0",
+        endpoint_url="http://node:8000",
+        trace_hash_ids=(11, 22, 33),
+    )
+    payload = json.dumps(asdict(m), sort_keys=True)
+    parsed = json.loads(payload)
+    assert parsed["t_dispatch_unix"] == 1700000000.123
+    assert parsed["t_first_token_unix"] == 1700000000.5
+    assert parsed["t_finish_unix"] == 1700000002.0
+    assert parsed["proxy_request_id"] == "sess1:0:42:0"
+    assert parsed["endpoint_url"] == "http://node:8000"
+    assert parsed["trace_hash_ids"] == [11, 22, 33]
+
+
+def test_proxy_request_id_matches_replayer_request_id():
+    m = _make_metric(proxy_request_id="sess1:0:42:0")
+    assert m.proxy_request_id == m.request_id