pageserver: add metric for number of zeroed pages on rel extend (#9492)

## Problem

Filling the gap in with zeroes is annoying for sharded ingest. We are
not sure it even happens in reality.

## Summary of Changes

Add one global counter which tracks how many such gap blocks we filled
on relation extends. We can add more metrics once we understand the
scope.

pageserver/src/metrics.rs

@@ -2092,6 +2092,7 @@ pub(crate) struct WalIngestMetrics {
     pub(crate) records_received: IntCounter,
     pub(crate) records_committed: IntCounter,
     pub(crate) records_filtered: IntCounter,
+    pub(crate) gap_blocks_zeroed_on_rel_extend: IntCounter,
 }
 
 pub(crate) static WAL_INGEST: Lazy<WalIngestMetrics> = Lazy::new(|| WalIngestMetrics {
@@ -2115,6 +2116,11 @@ pub(crate) static WAL_INGEST: Lazy<WalIngestMetrics> = Lazy::new(|| WalIngestMet
         "Number of WAL records filtered out due to sharding"
     )
     .expect("failed to define a metric"),
+    gap_blocks_zeroed_on_rel_extend: register_int_counter!(
+        "pageserver_gap_blocks_zeroed_on_rel_extend",
+        "Total number of zero gap blocks written on relation extends"
+    )
+    .expect("failed to define a metric"),
 });
 
 pub(crate) static WAL_REDO_TIME: Lazy<Histogram> = Lazy::new(|| {

pageserver/src/walingest.rs

@@ -1915,7 +1915,9 @@ impl WalIngest {
             modification.put_rel_extend(rel, new_nblocks, ctx).await?;
 
             let mut key = rel_block_to_key(rel, blknum);
+
             // fill the gap with zeros
+            let mut gap_blocks_filled: u64 = 0;
             for gap_blknum in old_nblocks..blknum {
                 key.field6 = gap_blknum;
 
@@ -1924,7 +1926,12 @@ impl WalIngest {
                 }
 
                 modification.put_rel_page_image_zero(rel, gap_blknum)?;
+                gap_blocks_filled += 1;
             }
+
+            WAL_INGEST
+                .gap_blocks_zeroed_on_rel_extend
+                .inc_by(gap_blocks_filled);
         }
         Ok(())
     }