feat(pageserver): ondemand download reason observability (#11780)

## Problem

Part of https://github.com/neondatabase/neon/issues/11615

## Summary of changes

We don't understand the root cause of why we get resident size surge
every now and then. This patch adds observability for that, and in the
next week, we might have a better understanding of what's going on.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/metrics.rs

@@ -497,6 +497,24 @@ pub(crate) static WAIT_LSN_IN_PROGRESS_GLOBAL_MICROS: Lazy<IntCounter> = Lazy::n
     .expect("failed to define a metric")
 });
 
+pub(crate) static ONDEMAND_DOWNLOAD_BYTES: Lazy<IntCounterVec> = Lazy::new(|| {
+    register_int_counter_vec!(
+        "pageserver_ondemand_download_bytes_total",
+        "Total bytes of layers on-demand downloaded",
+        &["task_kind"]
+    )
+    .expect("failed to define a metric")
+});
+
+pub(crate) static ONDEMAND_DOWNLOAD_COUNT: Lazy<IntCounterVec> = Lazy::new(|| {
+    register_int_counter_vec!(
+        "pageserver_ondemand_download_count",
+        "Total count of layers on-demand downloaded",
+        &["task_kind"]
+    )
+    .expect("failed to define a metric")
+});
+
 pub(crate) mod wait_ondemand_download_time {
     use super::*;
     const WAIT_ONDEMAND_DOWNLOAD_TIME_BUCKETS: &[f64] = &[

pageserver/src/tenant/storage_layer/layer.rs

@@ -4,6 +4,7 @@ use std::sync::{Arc, Weak};
 use std::time::{Duration, SystemTime};
 
 use crate::PERF_TRACE_TARGET;
+use crate::metrics::{ONDEMAND_DOWNLOAD_BYTES, ONDEMAND_DOWNLOAD_COUNT};
 use anyhow::Context;
 use camino::{Utf8Path, Utf8PathBuf};
 use pageserver_api::keyspace::KeySpace;
@@ -1255,6 +1256,14 @@ impl LayerInner {
 
                 self.access_stats.record_residence_event();
 
+                let task_kind: &'static str = ctx.task_kind().into();
+                ONDEMAND_DOWNLOAD_BYTES
+                    .with_label_values(&[task_kind])
+                    .inc_by(self.desc.file_size);
+                ONDEMAND_DOWNLOAD_COUNT
+                    .with_label_values(&[task_kind])
+                    .inc();
+
                 Ok(self.initialize_after_layer_is_on_disk(permit))
             }
             Err(e) => {