docs: Add Cayenne CDC and scan-path observability metrics

Document the new OpenTelemetry instruments registered for CDC
ingestion (burst duration, bytes, envelopes, fixed-cost phases)
and scan-path observability (listing table cache, fence wait,
scan duration) in the Cayenne deployment guide.

Author: claudespice

website/docs/components/data-accelerators/cayenne/deployment.md

@@ -72,7 +72,24 @@ Vortex compression typically delivers 2–4× better compression than Parquet Sn
 
 ## Metrics
 
-Generic acceleration metrics are available with the `dataset_acceleration_` prefix. Cayenne-specific OpenTelemetry instruments are not currently registered at the runtime layer; use Cayenne's footer/segment cache hit rates surfaced via query plan explain and the generic acceleration counters.
+Generic acceleration metrics are available with the `dataset_acceleration_` prefix. Cayenne also registers the following OpenTelemetry instruments for CDC ingestion and scan-path observability, all tagged by `dataset`:
+
+### CDC Apply Metrics
+
+| Metric | Type | Unit | Description |
+| ------ | ---- | ---- | ----------- |
+| `dataset_acceleration_cdc_apply_burst_duration_ms` | Histogram | ms | Duration to apply one coalesced CDC burst. |
+| `dataset_acceleration_cdc_apply_burst_bytes` | Histogram | bytes | Arrow in-memory bytes in one coalesced CDC apply burst. |
+| `dataset_acceleration_cdc_apply_burst_envelopes` | Histogram | envelopes | Number of source envelopes in one coalesced CDC apply burst. |
+| `dataset_acceleration_cdc_apply_fixed_cost_ms` | Histogram | ms | Duration for fixed-cost phases of CDC apply (with `phase` label: `finalize_wait`, `commit_wait`, etc.). |
+
+### Scan-Path Metrics
+
+| Metric | Type | Unit | Description |
+| ------ | ---- | ---- | ----------- |
+| `cayenne_scan_listing_table_cache_entries` | Gauge | entries | Number of entries in the scan `ListingTable` cache. Cleared on snapshot change (compaction/sort/overwrite). |
+| `cayenne_listing_fence_wait_duration_ms` | Histogram | ms | Time spent waiting on listing-fence reads during scans. |
+| `cayenne_listing_scan_duration_ms` | Histogram | ms | Duration of listing-table scans. |
 
 See [Component Metrics](../../../features/observability/component_metrics) for enabling and exporting metrics.