perf(hotpath): add fine-grained PUT-path stage guards (HP-14) (#4541)

Close the ~10ms instrumentation residual on the PUT success path that the
existing coarse writer_setup/encode/rename stage metrics do not attribute.
Adds `hp_guard!` measurement scopes to the previously uninstrumented
sub-stages called out in backlog#935 item 2:

- SetDisks::acquire_read_lock / acquire_write_lock (namespace lock acquire)
- S3::put_object_prelookup (pre-write get_object_info lookup)
- MultiWriter::shutdown (bitrot writer flush/close)
- SetDisks::commit_rename_data_dir (old data-dir reclaim)
- S3Access::put_object (S3 authorization segment)

Instrumentation only: `hp_guard!` expands to nothing without the `hotpath`
feature, so this is a pure-observation change with zero behavior impact and
zero cost in default builds. The pre-lookup site wraps only the lookup call
in a scoped block so the guard measures that slice exactly while preserving
the existing match and control flow.

Verified: cargo check -p rustfs-ecstore (default and --features hotpath) and
cargo check -p rustfs (default and --features hotpath) all pass.

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

crates/ecstore/src/erasure/coding/encode.rs

@@ -279,6 +279,7 @@ impl<'a> MultiWriter<'a> {
     }
 
     pub async fn shutdown(&mut self) -> std::io::Result<()> {
+        crate::hp_guard!("MultiWriter::shutdown");
         {
             let mut futures = FuturesUnordered::new();
             for (writer_opt, err) in self.writers.iter_mut().zip(self.errs.iter_mut()) {