feat(observability): feature-gated hotpath instrumentation for the data path (#4394)

Merge the hotpath-rs wall-time instrumentation from the backlog#936 analysis worktree behind an opt-in 'hotpath' cargo feature, keeping the default build at zero overhead and zero dependency.

- hotpath is an optional dependency everywhere (dep:hotpath feature syntax); the default dependency tree contains no hotpath crate at all
- 40+ measurement points across S3 handlers, ECStore/SetDisks object and multipart ops, erasure encode/decode, bitrot, LocalDisk I/O, FileMeta codec, and HashReader
- attribute sites use #[cfg_attr(feature = "hotpath", hotpath::measure)]; async_trait bodies use per-crate hp_guard! macros (ecstore + rustfs bin); rio gates measure_block! behind hp_measure_block!
- feature chain: rustfs -> rustfs-ecstore -> rustfs-rio / rustfs-filemeta, each crate owning its own gate
- hotpath-alloc is intentionally not wired up (hotpath 0.21.x TLS panic on cross-thread guard drop under tokio, see backlog#935); mimalloc stays the unconditional global allocator
- docs/development/hotpath-profiling.md documents building, HOTPATH_* env vars, SIGTERM report flow, and how to reproduce the backlog#936 timing reports

Refs: https://github.com/rustfs/backlog/issues/935 (HP-14, item 2), https://github.com/rustfs/backlog/issues/936

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

crates/filemeta/src/filemeta/codec.rs

@@ -19,6 +19,7 @@ impl FileMeta {
         !matches!(Self::check_xl2_v1(buf), Err(_e))
     }
 
+    #[cfg_attr(feature = "hotpath", hotpath::measure(impl_type = "FileMeta"))]
     pub fn load(buf: &[u8]) -> Result<FileMeta> {
         let mut xl = FileMeta::default();
         xl.unmarshal_msg(buf)?;
@@ -111,6 +112,7 @@ impl FileMeta {
         Ok((bin_len, remaining))
     }
 
+    #[cfg_attr(feature = "hotpath", hotpath::measure(impl_type = "FileMeta"))]
     pub fn unmarshal_msg(&mut self, buf: &[u8]) -> Result<u64> {
         let i = buf.len() as u64;
 
@@ -324,6 +326,7 @@ impl FileMeta {
         }
     }
 
+    #[cfg_attr(feature = "hotpath", hotpath::measure(impl_type = "FileMeta"))]
     pub fn marshal_msg(&self) -> Result<Vec<u8>> {
         let mut wr = Vec::new();