docs: fix dbStats example in contrib/database/sql (#2669)

contrib/database/sql/example_test.go

@@ -108,14 +108,16 @@ func Example_dbmPropagation() {
 }
 
 func Example_dbStats() {
-	sqltrace.Register("postgres", &pq.Driver{})
+	// Register the driver with the WithDBStats option to enable DBStats metric polling
+	sqltrace.Register("postgres", &pq.Driver{}, sqltrace.WithDBStats())
+	// Followed by a call to Open.
 	db, err := sqltrace.Open("postgres", "postgres://pqgotest:password@localhost/pqgotest?sslmode=disable")
 
 	if err != nil {
 		log.Fatal(err)
 	}
 
-	// Tracing is now enabled. Continue to use the database/sql package as usual
+	// Tracing and metric polling is now enabled. Metrics  will be submitted to Datadog with the prefix `datadog.tracer.sql`
 	rows, err := db.Query("SELECT name FROM users WHERE age=?", 27)
 	if err != nil {
 		log.Fatal(err)