pageserver: add vectored get latency histogram (#6461)

This patch introduces a new set of grafana metrics for a histogram:
pageserver_get_vectored_seconds_bucket{task_kind="Compaction|PageRequestHandler"}.

While it has a `task_kind` label, only compaction and SLRU fetches are
tracked. This reduces the increase in cardinality to 24.

The metric should allow us to isolate performance regressions while the
vectorized get is being implemented. Once the implementation is
complete, it'll also allow us to quantify the improvements.

pageserver/src/metrics.rs

@@ -150,6 +150,43 @@ pub(crate) static MATERIALIZED_PAGE_CACHE_HIT: Lazy<IntCounter> = Lazy::new(|| {
     .expect("failed to define a metric")
 });
 
+pub(crate) struct GetVectoredLatency {
+    map: EnumMap<TaskKind, Option<Histogram>>,
+}
+
+impl GetVectoredLatency {
+    // Only these task types perform vectored gets. Filter all other tasks out to reduce total
+    // cardinality of the metric.
+    const TRACKED_TASK_KINDS: [TaskKind; 2] = [TaskKind::Compaction, TaskKind::PageRequestHandler];
+
+    pub(crate) fn for_task_kind(&self, task_kind: TaskKind) -> Option<&Histogram> {
+        self.map[task_kind].as_ref()
+    }
+}
+
+pub(crate) static GET_VECTORED_LATENCY: Lazy<GetVectoredLatency> = Lazy::new(|| {
+    let inner = register_histogram_vec!(
+        "pageserver_get_vectored_seconds",
+        "Time spent in get_vectored",
+        &["task_kind"],
+        CRITICAL_OP_BUCKETS.into(),
+    )
+    .expect("failed to define a metric");
+
+    GetVectoredLatency {
+        map: EnumMap::from_array(std::array::from_fn(|task_kind_idx| {
+            let task_kind = <TaskKind as enum_map::Enum>::from_usize(task_kind_idx);
+
+            if GetVectoredLatency::TRACKED_TASK_KINDS.contains(&task_kind) {
+                let task_kind = task_kind.into();
+                Some(inner.with_label_values(&[task_kind]))
+            } else {
+                None
+            }
+        })),
+    }
+});
+
 pub(crate) struct PageCacheMetricsForTaskKind {
     pub read_accesses_materialized_page: IntCounter,
     pub read_accesses_immutable: IntCounter,

pageserver/src/tenant/timeline.rs

@@ -678,6 +678,10 @@ impl Timeline {
             return Err(GetVectoredError::Oversized(key_count));
         }
 
+        let _timer = crate::metrics::GET_VECTORED_LATENCY
+            .for_task_kind(ctx.task_kind())
+            .map(|t| t.start_timer());
+
         let mut values = BTreeMap::new();
         for range in key_ranges {
             let mut key = range.start;