maintain access stats for historic layers

This patch adds basic access statistics for historic layers
and exposes them in the management API's `LayerMapInfo`.

We record the accesses in the `{Delta,Image}Layer::load()` function
because it's the common path of
* page_service (`Timline::get_reconstruct_data()`)
* Compaction (`PersistentLayer::iter()` and `PersistentLayer::key_iter()`)

The stats survive residence status changes, and record these as well.

When scraping the layer map endpoint to record its evolution over time,
one must account for stat resets because they are in-memory only and
will reset on pageserver restart.
Use the launch timestamp header added by (#3527) to identify pageserver restarts.

This is PR https://github.com/neondatabase/neon/pull/3496

libs/pageserver_api/src/models.rs

@@ -1,9 +1,14 @@
-use std::num::{NonZeroU64, NonZeroUsize};
+use std::{
+    collections::HashMap,
+    num::{NonZeroU64, NonZeroUsize},
+    time::SystemTime,
+};
 
 use byteorder::{BigEndian, ReadBytesExt};
 use serde::{Deserialize, Serialize};
 use serde_with::{serde_as, DisplayFromStr};
 use utils::{
+    history_buffer::HistoryBufferWithDropCounter,
     id::{NodeId, TenantId, TimelineId},
     lsn::Lsn,
 };
@@ -233,6 +238,82 @@ pub struct LayerMapInfo {
     pub historic_layers: Vec<HistoricLayerInfo>,
 }
 
+#[derive(Debug, Hash, PartialEq, Eq, Clone, Copy, Serialize, Deserialize, enum_map::Enum)]
+#[repr(usize)]
+pub enum LayerAccessKind {
+    GetValueReconstructData,
+    Iter,
+    KeyIter,
+    Dump,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct LayerAccessStatFullDetails {
+    pub when_millis_since_epoch: u64,
+    pub task_kind: &'static str,
+    pub access_kind: LayerAccessKind,
+}
+
+/// An event that impacts the layer's residence status.
+#[serde_as]
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct LayerResidenceEvent {
+    /// The time when the event occurred.
+    /// NB: this timestamp is captured while the residence status changes.
+    /// So, it might be behind/ahead of the actual residence change by a short amount of time.
+    ///
+    #[serde(rename = "timestamp_millis_since_epoch")]
+    #[serde_as(as = "serde_with::TimestampMilliSeconds")]
+    timestamp: SystemTime,
+    /// The new residence status of the layer.
+    status: LayerResidenceStatus,
+    /// The reason why we had to record this event.
+    reason: LayerResidenceEventReason,
+}
+
+/// The reason for recording a given [`ResidenceEvent`].
+#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
+pub enum LayerResidenceEventReason {
+    /// The layer map is being populated, e.g. during timeline load or attach.
+    /// This includes [`RemoteLayer`] objects created in [`reconcile_with_remote`].
+    /// We need to record such events because there is no persistent storage for the events.
+    LayerLoad,
+    /// We just created the layer (e.g., freeze_and_flush or compaction).
+    /// Such layers are always [`LayerResidenceStatus::Resident`].
+    LayerCreate,
+    /// We on-demand downloaded or evicted the given layer.
+    ResidenceChange,
+}
+
+/// The residence status of the layer, after the given [`LayerResidenceEvent`].
+#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
+pub enum LayerResidenceStatus {
+    /// Residence status for a layer file that exists locally.
+    /// It may also exist on the remote, we don't care here.
+    Resident,
+    /// Residence status for a layer file that only exists on the remote.
+    Evicted,
+}
+
+impl LayerResidenceEvent {
+    pub fn new(status: LayerResidenceStatus, reason: LayerResidenceEventReason) -> Self {
+        Self {
+            status,
+            reason,
+            timestamp: SystemTime::now(),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Serialize)]
+pub struct LayerAccessStats {
+    pub access_count_by_access_kind: HashMap<LayerAccessKind, u64>,
+    pub task_kind_access_flag: Vec<&'static str>,
+    pub first: Option<LayerAccessStatFullDetails>,
+    pub accesses_history: HistoryBufferWithDropCounter<LayerAccessStatFullDetails, 16>,
+    pub residence_events_history: HistoryBufferWithDropCounter<LayerResidenceEvent, 16>,
+}
+
 #[serde_as]
 #[derive(Debug, Clone, Serialize)]
 #[serde(tag = "kind")]
@@ -262,6 +343,7 @@ pub enum HistoricLayerInfo {
         #[serde_as(as = "DisplayFromStr")]
         lsn_end: Lsn,
         remote: bool,
+        access_stats: LayerAccessStats,
     },
     Image {
         layer_file_name: String,
@@ -270,6 +352,7 @@ pub enum HistoricLayerInfo {
         #[serde_as(as = "DisplayFromStr")]
         lsn_start: Lsn,
         remote: bool,
+        access_stats: LayerAccessStats,
     },
 }