Add estore stats

[sggeorgiev]

Overview

Adds detailed per-estore statistics to help visualize and monitor the internal structure and distribution of items across ebuckets. These metrics provide insight into how data cascades across different TTL tiers (L1/L2/L3), aiding both debugging and TTL expiry tuning.

Changes

• estoreGetStats(): Collects and formats structural statistics for a given estore, with support for both standalone and clustered modes
• estoreCombineStats(): Aggregates statistics from multiple ebuckets instances
• ebGetStats(): Walks internal structures (stack, rax, list) and computes metrics like total items, segments, and level distribution
• ebGetStatsMsg(): Formats stats into human-readable text for use in CLI/debug output

Key Metrics Collected

• total items: Total keys stored
• total buckets: Total hash buckets in use
• total segments: Number of storage segments allocated
• total items in L1/L2/L3: Distribution across TTL cascade levels
• avg items per bucket: Data distribution efficiency
• avg items per segment: Segmentation density
• avg segments per bucket: Storage fragmentation indicator

Test Coverage

• ✅ estoreCombineStats() correctly aggregates and computes averages
• ✅ Handles edge case where totalBuckets == 0 (skips avg calculations)
• ✅ estoreGetStats() works in non-clustered mode, including L1/L2/L3 splits
• ✅ estoreGetStats() works in clustered mode, combining all bucket stats
• ✅ CLI output verified to contain all expected fields and values

Use Cases

• Cascade Monitoring: See how items are distributed across L1 (short TTL), L2 (medium TTL), and L3 (long TTL) tiers
• Expiration Behavior Analysis: Understand how TTLs shape storage patterns over time
• Debugging & Profiling: Identify imbalances or fragmentation in bucket and segment usage
• Capacity Planning: Track how data structures scale and adapt under different workloads

Example Usage (CLI)

127.0.0.1:6379> set test1 test ex 1000
OK
127.0.0.1:6379> set test2 test ex 6000
OK
127.0.0.1:6379> set test3 test ex 2000000000000
OK
127.0.0.1:6379> debug HTSTATS 0
[Dictionary HT]
Hash table 0 stats (main hash table):
 table size: 4
 number of elements: 3
[EBUCKETS]
Ebuckets stats:
 total items: 3
 total buckets: 2
 total segments: 2
 total items in L1: 1
 total items in L2: 1
 total items in L3: 1