kv: fix assertion in TestProxyTracing

This commit fixes the assertion in TestProxyTracing so that the test
will fail if request proxying is not working as expected. The test was
fooling itself, expecting QueryRowContext to return a nil Row if no
matching trace event was found. This is not the case, as a nil Row is
never returned. Instead, Row.Scan returns ErrNoRows if no matching row
is found.

Also, the query wasn't even running because it was passing the last line
of the query string in as a parameter, leading to the error: "pq: got 1
parameters but the statement requires 0".

I confirmed that before this change, the test passes even with request
proxying disabled. After this change, the test fails.

Epic: None
Release note: None

pkg/kv/kvclient/kvcoord/dist_sender_server_test.go

@@ -8,6 +8,7 @@ package kvcoord_test
 import (
 	"bytes"
 	"context"
+	gosql "database/sql"
 	"fmt"
 	"reflect"
 	"sort"
@@ -4898,11 +4899,17 @@ func TestProxyTracing(t *testing.T) {
 
 	// Expect the "proxy request complete" message to be in the trace and that it
 	// comes from the proxy node n2.
-	row := conn.QueryRowContext(ctx, "SELECT message, tag, location "+
-		"FROM [SHOW TRACE FOR SESSION] "+
-		"WHERE message LIKE '%proxy request complete%'"+
-		"AND location LIKE '%server/node%'",
-		"AND tag LIKE '%n2%'",
-	)
-	require.NotNil(t, row)
+	var msg, tag, loc string
+	if err = conn.QueryRowContext(ctx, `SELECT message, tag, location
+		FROM [SHOW TRACE FOR SESSION]
+		WHERE message LIKE '%proxy request complete%'
+		AND location LIKE '%server/node%'
+		AND tag LIKE '%n2%'`,
+	).Scan(&msg, &tag, &loc); err != nil {
+		if errors.Is(err, gosql.ErrNoRows) {
+			t.Fatalf("request succeeded without proxying")
+		}
+		t.Fatal(err)
+	}
+	t.Logf("found trace event; msg=%s, tag=%s, loc=%s", msg, tag, loc)
 }