feat(metrics): export per-level SST file counts to Prometheus

Adds rocksdb_num_files_at_level{db, level} gauge, polled every 5s
via the existing stats exporter loop. Covers levels 0-6 (RocksDB default).
Useful for monitoring L0 file accumulation during initial sync.

Author: philippem

src/new_index/db.rs

@@ -255,7 +255,9 @@ impl DB {
 
     pub fn start_stats_exporter(&self, db_metrics: Arc<RocksDbMetrics>, db_name: &str) {
         let db_arc = Arc::clone(&self.db);
+        let db_arc_levels = Arc::clone(&self.db);
         let label = db_name.to_string();
+        let label_levels = label.clone();
 
         let update_gauge = move |gauge: &GaugeVec, property: &str| {
             if let Ok(Some(value)) = db_arc.property_value(property) {
@@ -299,6 +301,17 @@ impl DB {
             update_gauge(&db_metrics.block_cache_capacity, "rocksdb.block-cache-capacity");
             update_gauge(&db_metrics.block_cache_usage, "rocksdb.block-cache-usage");
             update_gauge(&db_metrics.block_cache_pinned_usage, "rocksdb.block-cache-pinned-usage");
+            for level in 0..7u32 {
+                let prop = format!("rocksdb.num-files-at-level{}", level);
+                let level_str = level.to_string();
+                if let Ok(Some(value)) = db_arc_levels.property_value(&prop) {
+                    if let Ok(v) = value.parse::<f64>() {
+                        db_metrics.num_files_at_level
+                            .with_label_values(&[&label_levels, &level_str])
+                            .set(v);
+                    }
+                }
+            }
             thread::sleep(Duration::from_secs(5));
         });
     }

src/new_index/db_metrics.rs

@@ -49,6 +49,7 @@ pub struct RocksDbMetrics {
 
     // Level metrics
     pub base_level: GaugeVec,
+    pub num_files_at_level: GaugeVec,
 
     // Write metrics
     pub actual_delayed_write_rate: GaugeVec,
@@ -204,6 +205,10 @@ impl RocksDbMetrics {
                 format!("rocksdb_base_level"),
                 "Base level for compaction."
             ), labels),
+            num_files_at_level: metrics.gauge_vec(MetricOpts::new(
+                "rocksdb_num_files_at_level",
+                "Number of SST files at each compaction level."
+            ), &["db", "level"]),
 
             // Write metrics
             actual_delayed_write_rate: metrics.gauge_vec(MetricOpts::new(