feat(reviewer): surface merge-decision metrics (wire export_metrics_json)

.console/log.md

@@ -7001,3 +7001,19 @@ detect_regression/generate_alerts/save_snapshot/save_alert from d12_baseline —
 D12 gate confirms 0. (calculate_trend_slope/volatility/get_historical_data and
 categorize_alert/get_routes_for_alert remain genuinely unwired public API — stay
 baselined.) Observer suite 1389 green; ruff+ty+audit(B2)+doctor clean.
+
+## 2026-06-18 — COMPLETE merge-decision metrics: surface export_metrics_json
+
+Observer-plane #313 remediation. Investigation corrected the premise: the audit
+flagged MergeDecisionInstrumenter/DecisionMetricsCollector as "never
+instantiated", but they're ALREADY wired — pr_review_watcher calls the module-
+level record_decision_outcome at 5 decision points → get_instrumenter() →
+MergeDecisionInstrumenter records every merge decision. The genuine gap was
+narrow: export_metrics_json / get_metrics_summary had NO caller, so the
+collected metrics went nowhere. Wired it: _export_decision_metrics(status_dir)
+writes get_instrumenter().export_metrics_json() to status_dir/
+merge_decision_metrics.json each poll cycle (alongside the heartbeat),
+best-effort. Pruned export_metrics_json from d12_baseline (D12 gate confirms 0).
+1 test; reviewer suite 113 green (tests/ root + the #322 dedicated CI job);
+audit B2-env + doctor + D12 clean. Another false-positive corrected — the
+instrumenter wasn't unwired, only its export surface was.

.custodian/config.yaml

@@ -52,7 +52,6 @@ audit:
     - evaluate_alerts_dry_run
     - evaluate_per_collector_thresholds
     - export_all
-    - export_metrics_json
     - failure_reason_summary
     - filter_by_category
     - filter_by_extraction_status

src/operations_center/entrypoints/pr_review_watcher/main.py

@@ -57,6 +57,7 @@
     close_without_receipt_allowed,
 )
 from operations_center.reviewer.instrumentation import (
+    get_instrumenter,
     record_decision_outcome,
     record_ci_gate_defer,
     record_escalation,
@@ -2512,6 +2513,22 @@ def _write_heartbeat(status_dir: Path) -> None:
         pass
 
 
+def _export_decision_metrics(status_dir: Path) -> None:
+    """Surface the merge-decision metrics the instrumenter collects.
+
+    pr_review_watcher records every merge decision via record_decision_outcome
+    (→ the global MergeDecisionInstrumenter), but the collected metrics had no
+    reader: export_metrics_json was never called. Write the instrumenter's
+    summary to the status dir each cycle so the metrics are actually observable.
+    Best-effort — metrics export must never break the review loop."""
+    try:
+        status_dir.mkdir(parents=True, exist_ok=True)
+        metrics_path = status_dir / "merge_decision_metrics.json"
+        metrics_path.write_text(get_instrumenter().export_metrics_json(), encoding="utf-8")
+    except Exception:
+        pass
+
+
 # ── Poll cycle ────────────────────────────────────────────────────────────────
 
 
@@ -2643,6 +2660,7 @@ def main() -> int:
             logger.error("pr_review_watcher: error — %s", exc, exc_info=True)
             return 1
         _write_heartbeat(status_dir)
+        _export_decision_metrics(status_dir)
         return 0
 
     logger.info("pr_review_watcher: starting — poll_interval=%ds", args.poll_interval)
@@ -2653,6 +2671,7 @@ def main() -> int:
         except Exception as exc:
             logger.error("pr_review_watcher: unhandled error — %s", exc, exc_info=True)
         _write_heartbeat(status_dir)
+        _export_decision_metrics(status_dir)
         time.sleep(args.poll_interval)
 
 

tests/test_pr_review_watcher.py

@@ -1284,6 +1284,20 @@ def test_write_heartbeat_creates_file(tmp_path: Path) -> None:
     assert data["status"] == "active"
 
 
+def test_export_decision_metrics_writes_instrumenter_summary(tmp_path: Path) -> None:
+    # The reviewer records every merge decision via record_decision_outcome (→ the
+    # global MergeDecisionInstrumenter); _export_decision_metrics surfaces those
+    # collected metrics to the status dir (previously export_metrics_json had no
+    # caller — the metrics went nowhere).
+    watcher.record_decision_outcome(pr_number=7, repo_key="r", outcome="merge", reason="lgtm")
+    watcher._export_decision_metrics(tmp_path)
+    metrics_file = tmp_path / "merge_decision_metrics.json"
+    assert metrics_file.exists()
+    data = json.loads(metrics_file.read_text())
+    assert "total_decisions" in data and "outcomes" in data
+    assert data["total_decisions"] >= 1
+
+
 def test_write_heartbeat_idempotent(tmp_path: Path) -> None:
     watcher._write_heartbeat(tmp_path)
     watcher._write_heartbeat(tmp_path)