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

@@ -368,6 +368,7 @@ impl Erasure {
         Ok((reader, total))
     }
 
+    #[cfg_attr(feature = "hotpath", hotpath::measure)]
     pub async fn encode<R>(
         self: Arc<Self>,
         mut reader: R,
@@ -510,6 +511,7 @@ impl Erasure {
         Ok((reader, total))
     }
 
+    #[cfg_attr(feature = "hotpath", hotpath::measure)]
     pub async fn encode_batched<R>(
         self: Arc<Self>,
         mut reader: R,
@@ -637,6 +639,7 @@ impl Erasure {
 
     /// Fast path for small inline objects: skip tokio::spawn + mpsc channel.
     /// Reads all data, encodes directly, writes shards sequentially.
+    #[cfg_attr(feature = "hotpath", hotpath::measure)]
     pub async fn encode_inline_small<R>(
         self: Arc<Self>,
         reader: R,
@@ -651,6 +654,7 @@ impl Erasure {
 
     /// Fast path for single-block non-inline objects: avoids the producer/consumer
     /// pipeline in `encode()` while keeping the same writer/quorum/shutdown semantics.
+    #[cfg_attr(feature = "hotpath", hotpath::measure)]
     pub async fn encode_single_block_non_inline<R>(
         self: Arc<Self>,
         reader: R,