fix(obs): hide unwired request metrics (#4481)

Refs rustfs/backlog#1006

- aggregate request traffic samples per type to avoid future counter collisions
- keep request schema and collector crate-internal until a production stats source exists
- preserve focused regression coverage for the internal request collector logic

Co-authored-by: heihutu <heihutu@gmail.com>

crates/obs/src/metrics/schema/mod.rs

@@ -30,7 +30,7 @@ pub mod node_disk;
 pub mod notification_target;
 pub mod process_resource;
 pub mod replication;
-pub mod request;
+pub(crate) mod request;
 pub mod scanner;
 pub mod system_cpu;
 pub mod system_drive;