Add feature-gated Rust heap usage tracking for Valkey modules

[misaki112]

Summary

Adds optional per-module Rust heap usage tracking through the existing ValkeyAlloc allocator.

The new enable-usage-tracking feature exposes valkey_module::module_stats::snapshot() with:

• Current and peak tracked bytes
• Allocation and free counts
• Derived live-allocation count
• Saturating-accounting error count

Implementation

• Tracks alloc, alloc_zeroed, and dealloc calls using aligned allocation sizes.
• Uses saturating arithmetic to prevent counter underflow or overflow.
• Adds the usage_tracking example module with commands to retain and release:
  • Vec<u8>
  • DashMap
  • boxed AtomicU64 storage
  • String
  • a custom heap-owning struct
• Adds integration coverage confirming each allocation type increases tracked memory and returns near baseline after release.
• Verifies Redis/Valkey keyspace memory is not attributed to module usage tracking.
• Documents feature enablement, available metrics, and the realloc limitation in the README.

Limitation

GlobalAlloc::realloc is intentionally not tracked. Resize-heavy Vec and String paths can therefore temporarily overstate peak memory and make allocation counts noisier.

Validation

cargo test --lib module_stats
cargo build --example usage_tracking --features enable-usage-tracking
cargo test --features enable-usage-tracking --test integration test_usage_tracking_module_memory -- --exact
cargo check --features enable-usage-tracking --all-targets
cargo fmt --check

Potential Follow-up

This feature flag may be extended in the future to include additional per-module resource observability, such as CPU-usage metrics.