sql: add notice for custom var with the same name as a cluster setting

No release note since custom session variables are new in 22.1.

Release note: None

pkg/sql/logictest/testdata/logic_test/set

@@ -753,6 +753,13 @@ SHOW tracing.custom
 ----
 ijk
 
+# Show a notice if a custom session variable has the same name as a cluster setting.
+query T noticetrace
+SET server.shutdown.drain_wait = '10s'
+----
+NOTICE: setting custom variable "server.shutdown.drain_wait"
+HINT: did you mean SET CLUSTER SETTING?
+
 # Test that RESET ALL changes custom options to empty strings.
 statement ok
 RESET ALL

pkg/sql/set_var.go

@@ -17,6 +17,7 @@ import (
 
 	"github.com/cockroachdb/apd/v3"
 	"github.com/cockroachdb/cockroach/pkg/server/telemetry"
+	"github.com/cockroachdb/cockroach/pkg/settings"
 	"github.com/cockroachdb/cockroach/pkg/sql/paramparse"
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgcode"
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgerror"
@@ -61,6 +62,15 @@ func (p *planner) SetVar(ctx context.Context, n *tree.SetVar) (planNode, error)
 	if err != nil {
 		return nil, err
 	}
+	if _, ok := settings.Lookup(name, settings.LookupForLocalAccess, p.ExecCfg().Codec.ForSystemTenant()); ok {
+		p.BufferClientNotice(
+			ctx,
+			errors.WithHint(
+				pgnotice.Newf("setting custom variable %q", name),
+				"did you mean SET CLUSTER SETTING?",
+			),
+		)
+	}
 
 	var typedValues []tree.TypedExpr
 	if len(n.Values) > 0 {