docs: update LATENCY PR description to reflect review fixes

Author: gongna-au

LATENCY_PR_DESCRIPTION.md

@@ -0,0 +1,61 @@
+# PR Description: feat(server): implement LATENCY command set (Redis 7.0+ compatibility)
+
+## Summary
+This PR implements the `LATENCY` command for Kvrocks, focusing on the `LATENCY HISTOGRAM` subcommand introduced in Redis 7.0. It leverages the existing histogram infrastructure (`histogram-bucket-boundaries` config + `CommandHistogram` stats) to expose per-command latency distributions via the standard Redis protocol.
+
+## Changes
+### New File: `src/commands/cmd_latency.cc`
+
+Implements `CommandLatency` with three subcommands:
+
+- **`LATENCY HISTOGRAM [command ...]`** — Returns a cumulative distribution of command latencies.
+  - Output uses the Redis 7.0 format: outer map keyed by command name, each value is a map with `calls` (integer) and `histogram_usec` (map of boundary → cumulative count).
+  - Frequency counts from `CommandHistogram::buckets` are converted to cumulative sums at output time.
+  - Empty leading buckets are skipped (matching Redis behavior of omitting zero-count ranges).
+  - The last bucket (overflow) is rendered as `"inf"`.
+  - Returns an empty map if `histogram-bucket-boundaries` is not configured.
+
+- **`LATENCY RESET [event ...]`** — Returns `(integer) 0`.
+  - In Redis, this resets the spike-sampling event system (`LATENCY LATEST`/`HISTORY`/`GRAPH`), which Kvrocks does not have. Per Redis docs, histogram data is reset via `CONFIG RESETSTAT`, not `LATENCY RESET`. Returning 0 correctly indicates no event classes were reset.
+
+- **`LATENCY HELP`** — Returns usage information for all supported subcommands.
+
+### Build System
+No changes needed. The file is automatically picked up by `file(GLOB_RECURSE KVROCKS_SRCS src/*.cc)` in root `CMakeLists.txt`.
+
+### Command Registration
+Registered under `CommandCategory::Server` via `REDIS_REGISTER_COMMANDS(Server, ...)`, same category as `cmd_server.cc`. This is safe because the macro generates unique static variables per translation unit using `__LINE__`.
+
+## Compatibility
+
+| Subcommand | Redis | Kvrocks | Note |
+| --- | --- | --- | --- |
+| `LATENCY HISTOGRAM [cmd ...]` | 7.0+ | **Yes** | Cumulative distribution, `histogram_usec` field, empty buckets skipped |
+| `LATENCY HELP` | 7.0+ | **Yes** | |
+| `LATENCY RESET [event ...]` | 2.8.13+ | **Yes** | Always returns 0 (no spike-sampling infrastructure) |
+| `LATENCY LATEST` | 2.8.13+ | No | Requires spike-sampling event system |
+| `LATENCY HISTORY event` | 2.8.13+ | No | Requires spike-sampling event system |
+| `LATENCY GRAPH event` | 2.8.13+ | No | Requires spike-sampling event system |
+| `LATENCY DOCTOR` | 2.8.13+ | No | Requires spike-sampling event system |
+
+## Configuration Prerequisite
+Histograms are only tracked when `histogram-bucket-boundaries` is set in `kvrocks.conf`. Example:
+```
+histogram-bucket-boundaries 1,5,10,25,50,100,250,500,1000,2500,5000,10000
+```
+If empty (default), `LATENCY HISTOGRAM` returns an empty map — this is expected behavior, not an error.
+
+## RESP Protocol Details
+- Uses RESP3 Map (`%`) via `conn->HeaderOfMap()`, which auto-degrades to interleaved RESP2 arrays.
+- Per-command entry structure:
+  ```
+  command_name => {
+    "calls"          => (integer) N,
+    "histogram_usec" => {
+      boundary_1 => (integer) cumulative_count_1,
+      boundary_2 => (integer) cumulative_count_2,
+      ...
+      "inf"      => (integer) total_calls
+    }
+  }
+  ```