add global metric for unexpected on-demand downloads (#4069)

Until we have toned down the prod logs to zero WARN and ERROR, we want a
dedicated metric for which we can have a dedicated alert.

fixes https://github.com/neondatabase/neon/issues/3924

pageserver/src/bin/pageserver.rs

@@ -226,6 +226,7 @@ fn start_pageserver(
     );
     set_build_info_metric(GIT_VERSION);
     set_launch_timestamp_metric(launch_ts);
+    pageserver::preinitialize_metrics();
 
     // If any failpoints were set from FAILPOINTS environment variable,
     // print them to the log for debugging purposes

pageserver/src/lib.rs

@@ -44,6 +44,8 @@ pub const DELTA_FILE_MAGIC: u16 = 0x5A61;
 
 static ZERO_PAGE: bytes::Bytes = bytes::Bytes::from_static(&[0u8; 8192]);
 
+pub use crate::metrics::preinitialize_metrics;
+
 pub async fn shutdown_pageserver(exit_code: i32) {
     // Shut down the libpq endpoint task. This prevents new connections from
     // being accepted.

pageserver/src/metrics.rs

@@ -205,6 +205,15 @@ static EVICTIONS_WITH_LOW_RESIDENCE_DURATION: Lazy<IntCounterVec> = Lazy::new(||
     .expect("failed to define a metric")
 });
 
+pub static UNEXPECTED_ONDEMAND_DOWNLOADS: Lazy<IntCounter> = Lazy::new(|| {
+    register_int_counter!(
+        "pageserver_unexpected_ondemand_downloads_count",
+        "Number of unexpected on-demand downloads. \
+         We log more context for each increment, so, forgo any labels in this metric.",
+    )
+    .expect("failed to define a metric")
+});
+
 /// Each [`Timeline`]'s  [`EVICTIONS_WITH_LOW_RESIDENCE_DURATION`] metric.
 #[derive(Debug)]
 pub struct EvictionsWithLowResidenceDuration {
@@ -1132,3 +1141,10 @@ impl<F: Future<Output = Result<O, E>>, O, E> Future for MeasuredRemoteOp<F> {
         poll_result
     }
 }
+
+pub fn preinitialize_metrics() {
+    // We want to alert on this metric increasing.
+    // Initialize it eagerly, so that our alert rule can distinguish absence of the metric from metric value 0.
+    assert_eq!(UNEXPECTED_ONDEMAND_DOWNLOADS.get(), 0);
+    UNEXPECTED_ONDEMAND_DOWNLOADS.reset();
+}

pageserver/src/tenant/timeline.rs

@@ -48,7 +48,7 @@ use crate::tenant::{
 
 use crate::config::PageServerConf;
 use crate::keyspace::{KeyPartitioning, KeySpace};
-use crate::metrics::TimelineMetrics;
+use crate::metrics::{TimelineMetrics, UNEXPECTED_ONDEMAND_DOWNLOADS};
 use crate::pgdatadir_mapping::LsnForTimestamp;
 use crate::pgdatadir_mapping::{is_rel_fsm_block_key, is_rel_vm_block_key};
 use crate::pgdatadir_mapping::{BlockNumber, CalculateLogicalSizeError};
@@ -2355,6 +2355,7 @@ impl Timeline {
                             id,
                             ctx.task_kind()
                         );
+                        UNEXPECTED_ONDEMAND_DOWNLOADS.inc();
                         timeline.download_remote_layer(remote_layer).await?;
                         continue 'layer_map_search;
                     }

test_runner/fixtures/metrics.py

@@ -53,6 +53,7 @@ PAGESERVER_GLOBAL_METRICS: Tuple[str, ...] = (
     "pageserver_storage_operations_seconds_global_count",
     "pageserver_storage_operations_seconds_global_sum",
     "pageserver_storage_operations_seconds_global_bucket",
+    "pageserver_unexpected_ondemand_downloads_count_total",
     "libmetrics_launch_timestamp",
     "libmetrics_build_info",
     "libmetrics_tracing_event_count_total",