safekeeper: add initialize_segment variant of safekeeper_wal_storage_operation_seconds (#9691)

## Problem

We don't have a metric capturing the latency of segment initialization.
This can be significant due to fsyncs.

## Summary of changes

Add an `initialize_segment` variant of
`safekeeper_wal_storage_operation_seconds`.

safekeeper/src/metrics.rs

@@ -55,7 +55,7 @@ pub static WRITE_WAL_SECONDS: Lazy<Histogram> = Lazy::new(|| {
 pub static FLUSH_WAL_SECONDS: Lazy<Histogram> = Lazy::new(|| {
     register_histogram!(
         "safekeeper_flush_wal_seconds",
-        "Seconds spent syncing WAL to a disk",
+        "Seconds spent syncing WAL to a disk (excluding segment initialization)",
         DISK_FSYNC_SECONDS_BUCKETS.to_vec()
     )
     .expect("Failed to register safekeeper_flush_wal_seconds histogram")

safekeeper/src/wal_storage.rs

@@ -257,6 +257,9 @@ impl PhysicalStorage {
             // Try to open existing partial file
             Ok((file, true))
         } else {
+            let _timer = WAL_STORAGE_OPERATION_SECONDS
+                .with_label_values(&["initialize_segment"])
+                .start_timer();
             // Create and fill new partial file
             //
             // We're using fdatasync during WAL writing, so file size must not
@@ -274,8 +277,6 @@ impl PhysicalStorage {
             });
             file.set_len(self.wal_seg_size as u64).await?;
 
-            // Note: this doesn't get into observe_flush_seconds metric. But
-            // segment init should be separate metric, if any.
             if let Err(e) = durable_rename(&tmp_path, &wal_file_partial_path, !self.no_sync).await {
                 // Probably rename succeeded, but fsync of it failed. Remove
                 // the file then to avoid using it.