feat(metrics): export initial sync height as Prometheus gauge

Add initial_sync_height and initial_sync_progress_pct gauges, updated
after every batch so sync progress is visible in Prometheus without
waiting for the full update_blocks() call to complete.

Author: philippem

src/metrics.rs

@@ -48,6 +48,12 @@ impl Metrics {
         g
     }
 
+    pub fn float_gauge(&self, opts: prometheus::Opts) -> prometheus::Gauge {
+        let g = prometheus::Gauge::with_opts(opts).unwrap();
+        self.reg.register(Box::new(g.clone())).unwrap();
+        g
+    }
+
     pub fn gauge_vec(&self, opts: prometheus::Opts, labels: &[&str]) -> GaugeVec {
         let g = GaugeVec::new(opts, labels).unwrap();
         self.reg.register(Box::new(g.clone())).unwrap();

src/new_index/schema.rs

@@ -198,6 +198,8 @@ pub struct Indexer {
     iconfig: IndexerConfig,
     duration: HistogramVec,
     tip_metric: Gauge,
+    sync_height: Gauge,
+    sync_progress: prometheus::Gauge,
 }
 
 struct IndexerConfig {
@@ -245,6 +247,14 @@ impl Indexer {
                 &["step"],
             ),
             tip_metric: metrics.gauge(MetricOpts::new("tip_height", "Current chain tip height")),
+            sync_height: metrics.gauge(MetricOpts::new(
+                "initial_sync_height",
+                "Height of the last block batch completed during initial sync",
+            )),
+            sync_progress: metrics.float_gauge(MetricOpts::new(
+                "initial_sync_progress_pct",
+                "Initial sync progress as a percentage of the best known chain height",
+            )),
         }
     }
 
@@ -394,6 +404,13 @@ impl Indexer {
                     self.index(&to_index);
                 }
             }
+            if let Some(last) = blocks.last() {
+                let h = last.entry.height();
+                self.sync_height.set(h as i64);
+                if chain_tip_height > 0 {
+                    self.sync_progress.set(h as f64 / chain_tip_height as f64 * 100.0);
+                }
+            }
         });
 
         // Compact after all add+index work is done, not between passes.