P2.8: Verify middleware implementation - structured logging + Prometheus metrics + request IDs

Verified that the existing middleware implementation fully satisfies all acceptance
criteria for P2.8:

- Request ID generation (UUIDv7 prefix short-hashed) attached as X-Request-Id
- Structured JSON log per plan §10 shape with request_id trace correlation
- Prometheus metrics: request_duration_seconds, requests_total, requests_in_flight
- Scatter metrics: fan_out_size, partial_responses_total, retries_total
- Node metrics: node_healthy, node_request_duration_seconds, node_errors_total
- Metrics server on :9090 with /metrics endpoint
- High-cardinality defense using MatchedPath extractor for path_template

All acceptance tests passing:
- test_all_core_metrics_registered - 18 core metrics verified
- test_json_logs_parseable_by_jq - JSON parsing verified
- test_request_id_response_header - X-Request-Id in responses verified
- test_request_id_appears_in_all_log_lines_within_request - trace correlation verified

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

notes/miroir-9dj.8.md

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+# P2.8 Middleware: structured logging + prometheus metrics + request IDs
+
+## Summary
+
+Verified that the existing middleware implementation in `crates/miroir-proxy/src/middleware.rs` fully satisfies all acceptance criteria for P2.8.
+
+## Implementation Status
+
+### Request ID Generation ✓
+- `request_id_middleware` generates RequestId using UUIDv7
+- Hashes UUIDv7 to 8-character hex ID for compactness
+- Adds `X-Request-Id` header to every response
+- Preserves existing valid request IDs from client
+- Replaces invalid request IDs with generated ones
+
+### Structured JSON Logging ✓
+- `telemetry_middleware` logs with tracing-subscriber JSON output
+- Log format includes: timestamp, level, target, message, pod_id, request_id, duration_ms, status, method, path_template
+- Search handler adds: index, node_count, estimated_hits, degraded
+- `with_current_span(true)` ensures request_id propagates to all child events
+
+### Prometheus Metrics ✓
+All required metrics are implemented:
+- `miroir_request_duration_seconds{method, path_template, status}` - HistogramVec
+- `miroir_requests_total{method, path_template, status}` - CounterVec
+- `miroir_requests_in_flight` - Gauge with InFlightGuard for proper decrement
+- `miroir_scatter_fan_out_size` - Histogram
+- `miroir_scatter_partial_responses_total` - Counter
+- `miroir_scatter_retries_total` - Counter
+- `miroir_node_healthy{node_id}` - GaugeVec
+- `miroir_node_request_duration_seconds{node_id, operation}` - HistogramVec
+- `miroir_node_errors_total{node_id, error_type}` - CounterVec
+
+### Metrics Server ✓
+- Separate axum listener on port 9090
+- `/metrics` endpoint returns Prometheus exposition format
+- No auth (bound to cluster network)
+
+### High-Cardinality Defense ✓
+- Uses Axum's `MatchedPath` extractor to get route template
+- Example: `/indexes/{uid}/search` instead of `/indexes/products/search`
+- Prevents high-cardinality labels from filling Prometheus
+
+## Test Results
+
+All tests passing:
+- `test_all_core_metrics_registered` - Verifies all 18 core metrics are registered
+- `test_json_logs_parseable_by_jq` - Verifies log lines are valid JSON
+- `test_request_id_response_header` - Verifies X-Request-Id in responses
+- `test_request_id_appears_in_all_log_lines_within_request` - Verifies trace correlation
+- All 13 middleware lib tests passing
+- All 17 structured logging integration tests passing
+- All 5 core metrics tests passing
+
+## Files Verified
+
+- `crates/miroir-proxy/src/middleware.rs` - Core middleware implementation
+- `crates/miroir-proxy/src/main.rs` - Metrics server setup on :9090
+- `crates/miroir-proxy/src/routes/search.rs` - Search handler logging with plan §10 fields
+- `crates/miroir-proxy/tests/p7_1_core_metrics.rs` - Core metrics acceptance tests
+- `crates/miroir-proxy/tests/p7_5_structured_logging.rs` - Structured logging acceptance tests