fix: config api and export metrics default database (#4633)

src/servers/src/export_metrics.rs

@@ -59,7 +59,7 @@ pub struct SelfImportOption {
 impl Default for SelfImportOption {
     fn default() -> Self {
         Self {
-            db: "information_schema".to_string(),
+            db: "greptime_metrics".to_string(),
         }
     }
 }