feat(metrics): add batch_total timer to index_duration histogram

Add a batch_total label to the existing index_duration histogram that
covers the full wall-clock time of each add+index cycle. This makes it
easy to see per-batch throughput at a glance without summing the
individual step histograms (add_process, add_write, index_lookup,
index_process).

Only recorded for batches where actual work is done (blocks not yet
added or indexed), so no-op iterations do not skew the distribution.

Author: philippem

src/new_index/schema.rs

@@ -374,9 +374,6 @@ impl Indexer {
                     .cloned()
                     .collect()
             };
-            if !to_add.is_empty() {
-                self.add(&to_add);
-            }
 
             // Index blocks not yet in history (O rows for to_add are now in the write buffer)
             let to_index: Vec<_> = {
@@ -387,8 +384,15 @@ impl Indexer {
                     .cloned()
                     .collect()
             };
-            if !to_index.is_empty() {
-                self.index(&to_index);
+
+            if !to_add.is_empty() || !to_index.is_empty() {
+                let _batch_timer = self.start_timer("batch_total");
+                if !to_add.is_empty() {
+                    self.add(&to_add);
+                }
+                if !to_index.is_empty() {
+                    self.index(&to_index);
+                }
             }
         });