feat: refactor ratelimit to ratelimiter and improve BBR adaptive rate limiter

[gaius-qi]

• Rename ratelimit module to ratelimiter for clearer naming convention.
• Refactor BBR rolling window to remove per-sample timestamps and rely on ring buffer eviction for expiration, simplifying Sample struct and get_stats logic.
• Add bucket_count field and accessor to RollingWindow for use in estimated limit calculation.
• Integrate OverloadCollector with container-aware CPU/memory sampling and background collection loop with graceful shutdown support.
• Add RequestGuard RAII pattern for automatic metric recording and in-flight counter management.
• Move container detection logic from cpu.rs/memory.rs cgroup methods up to OverloadCollector, removing redundant is_running_in_container checks in get_cgroup_stats.
• Reduce CPU cgroup refresh interval from 1s to 500ms for more responsive overload detection.
• Use Self constructor shorthand in GC::new.

Description

This pull request introduces several improvements and code cleanups across the dragonfly-client-util and dragonfly-client crates. The most significant changes involve renaming a module for clarity, optimizing CPU refresh intervals, and simplifying cgroup resource checks for CPU and memory statistics.

Module renaming and code cleanup:

• Renamed the ratelimit module to ratelimiter in dragonfly-client-util/src/lib.rs for consistency and clarity.

Resource statistics logic simplification:

• Removed unnecessary container environment checks (is_running_in_container) from cgroup CPU and memory statistics retrieval, allowing stats to be fetched regardless of container status in dragonfly-client-util/src/sysinfo/cpu.rs and dragonfly-client-util/src/sysinfo/memory.rs. [1] [2]

Performance improvements:

• Reduced the default CPU refresh interval from 1 second to 500 milliseconds in dragonfly-client-util/src/sysinfo/cpu.rs for more frequent updates.

Minor code style improvements:

• Changed struct initialization in dragonfly-client/src/gc/mod.rs to use Self instead of explicit struct name for idiomatic Rust style.

Related Issue

Motivation and Context

Screenshots (if appropriate)

[codecov]

Codecov Report

❌ Patch coverage is 64.26799% with 144 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.44%. Comparing base (035ae91) to head (822ea35).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
dragonfly-client-util/src/ratelimiter/bbr.rs	64.42%	143 Missing ⚠️
dragonfly-client/src/gc/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1672      +/-   ##
==========================================
- Coverage   50.83%   50.44%   -0.39%     
==========================================
  Files          83       83              
  Lines       20029    20157     +128     
==========================================
- Hits        10182    10169      -13     
- Misses       9847     9988     +141     

Files with missing lines	Coverage Δ
dragonfly-client-util/src/sysinfo/cpu.rs	0.00% <ø> (ø)
dragonfly-client-util/src/sysinfo/memory.rs	0.00% <ø> (ø)
dragonfly-client/src/gc/mod.rs	0.00% <0.00%> (ø)
dragonfly-client-util/src/ratelimiter/bbr.rs	64.42% <64.42%> (ø)

... and 1 file with indirect coverage changes

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[EvanCley]

lgtm