fix(pageserver): categorize client error in basebackup metrics (#9110)

We separated client error from basebackup error log lines in
https://github.com/neondatabase/neon/pull/7523, but we didn't do
anything for the metrics. In this patch, we fixed it.

ref https://github.com/neondatabase/neon/issues/8970

## Summary of changes

We use the same criteria as in `log_query_error` producing an info line
(instead of error) for the metrics. We added a `client_error` category
for the basebackup query time metrics.

---------

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

libs/postgres_backend/src/lib.rs

@@ -984,6 +984,7 @@ pub fn short_error(e: &QueryError) -> String {
 }
 
 fn log_query_error(query: &str, e: &QueryError) {
+    // If you want to change the log level of a specific error, also re-categorize it in `BasebackupQueryTimeOngoingRecording`.
     match e {
         QueryError::Disconnected(ConnectionError::Io(io_error)) => {
             if is_expected_io_error(io_error) {

pageserver/src/metrics.rs

@@ -8,6 +8,8 @@ use metrics::{
 };
 use once_cell::sync::Lazy;
 use pageserver_api::shard::TenantShardId;
+use postgres_backend::{is_expected_io_error, QueryError};
+use pq_proto::framed::ConnectionError;
 use strum::{EnumCount, VariantNames};
 use strum_macros::{IntoStaticStr, VariantNames};
 use tracing::warn;
@@ -1508,6 +1510,7 @@ static COMPUTE_STARTUP_BUCKETS: Lazy<[f64; 28]> = Lazy::new(|| {
 pub(crate) struct BasebackupQueryTime {
     ok: Histogram,
     error: Histogram,
+    client_error: Histogram,
 }
 
 pub(crate) static BASEBACKUP_QUERY_TIME: Lazy<BasebackupQueryTime> = Lazy::new(|| {
@@ -1521,6 +1524,7 @@ pub(crate) static BASEBACKUP_QUERY_TIME: Lazy<BasebackupQueryTime> = Lazy::new(|
     BasebackupQueryTime {
         ok: vec.get_metric_with_label_values(&["ok"]).unwrap(),
         error: vec.get_metric_with_label_values(&["error"]).unwrap(),
+        client_error: vec.get_metric_with_label_values(&["client_error"]).unwrap(),
     }
 });
 
@@ -1557,7 +1561,7 @@ impl BasebackupQueryTime {
 }
 
 impl<'a, 'c> BasebackupQueryTimeOngoingRecording<'a, 'c> {
-    pub(crate) fn observe<T, E>(self, res: &Result<T, E>) {
+    pub(crate) fn observe<T>(self, res: &Result<T, QueryError>) {
         let elapsed = self.start.elapsed();
         let ex_throttled = self
             .ctx
@@ -1576,10 +1580,15 @@ impl<'a, 'c> BasebackupQueryTimeOngoingRecording<'a, 'c> {
                 elapsed
             }
         };
-        let metric = if res.is_ok() {
-            &self.parent.ok
-        } else {
-            &self.parent.error
+        // If you want to change categorize of a specific error, also change it in `log_query_error`.
+        let metric = match res {
+            Ok(_) => &self.parent.ok,
+            Err(QueryError::Disconnected(ConnectionError::Io(io_error)))
+                if is_expected_io_error(io_error) =>
+            {
+                &self.parent.client_error
+            }
+            Err(_) => &self.parent.error,
         };
         metric.observe(ex_throttled.as_secs_f64());
     }