Bound the other two PostgreSQL collectors that had no ceiling

PerformanceMonitor.Collectors/PgBufferUsageCollector.cs

@@ -135,6 +135,20 @@ HAVING count(*) >= 8
     /// </summary>
     public override bool AppliesTo(CollectorTargetInfo target) => true;
 
+    /// <summary>
+    /// Two minutes, because the COST here is the input scan and no LIMIT can reduce it.
+    ///
+    /// <para><c>pg_buffercache</c> materialises one row per shared buffer and takes buffer partition locks
+    /// while it does — on a large instance that is millions of rows, and the aggregate needs all of them,
+    /// so the usual trick of capping output does nothing. The output IS bounded (grouped per relation,
+    /// with an eight-buffer floor); the read is not.</para>
+    ///
+    /// <para>Without an override a slow scan ends as <c>Exception while reading from stream</c>, an
+    /// unclassified Npgsql failure — which is exactly how #2617 presented and why it went unnoticed for a
+    /// collector that had never once succeeded. A classified timeout says which collector was too slow.</para>
+    /// </summary>
+    public override int? CommandTimeoutSecondsOverride => 120;
+
     public override CollectorQuery BuildQuery(CollectorContext context) => new(QueryText);
 
     public override IReadOnlyList<CollectorColumn> PayloadColumns { get; } = new[]

PerformanceMonitor.Collectors/PgColumnStatsCollector.cs

@@ -119,7 +119,13 @@ JOIN pg_catalog.pg_namespace AS n
 WHERE s.schemaname NOT IN ('pg_catalog', 'information_schema')
 AND   c.relkind IN ('r', 'm', 'p')
 AND   c.relpages >= 128
-ORDER BY c.relpages DESC, s.schemaname, s.tablename, s.attname";
+ORDER BY c.relpages DESC, s.schemaname, s.tablename, s.attname
+/* Bounded (#2617's lesson applied before it bites). This is a catalog read rather than a page scan, so
+   it is nowhere near as costly as pg_index_bloat was - but it was still unbounded, and row count here is
+   tables x columns: 361 tables over the size floor on one measured target, and a wide schema turns that
+   into five figures per database per DAY. The ORDER BY already puts the biggest tables first, so the cap
+   keeps exactly the columns a plan-shape question is asked about. */
+LIMIT 5000";
 
     public override string Name => "pg_column_stats";