Feat: Benchmark CLI additions and documentation (#12536)

### What problem does this PR solve?

This PR adds a dedicated HTTP benchmark CLI for RAGFlow chat and
retrieval endpoints so we can measure latency/QPS.

### Type of change

- [x] Documentation Update
- [x] Other (please describe): Adds a CLI benchmarking tool for
chat/retrieval latency/QPS

---------

Co-authored-by: Liu An <asiro@qq.com>

test/benchmark/metrics.py

@@ -0,0 +1,67 @@
+import math
+from dataclasses import dataclass
+from typing import Any, List, Optional
+
+
+@dataclass
+class ChatSample:
+    t0: float
+    t1: Optional[float]
+    t2: Optional[float]
+    error: Optional[str] = None
+    response_text: Optional[str] = None
+
+    @property
+    def first_token_latency(self) -> Optional[float]:
+        if self.t1 is None:
+            return None
+        return self.t1 - self.t0
+
+    @property
+    def total_latency(self) -> Optional[float]:
+        if self.t2 is None:
+            return None
+        return self.t2 - self.t0
+
+
+@dataclass
+class RetrievalSample:
+    t0: float
+    t1: Optional[float]
+    error: Optional[str] = None
+    response: Optional[Any] = None
+
+    @property
+    def latency(self) -> Optional[float]:
+        if self.t1 is None:
+            return None
+        return self.t1 - self.t0
+
+
+def _percentile(sorted_values: List[float], p: float) -> Optional[float]:
+    if not sorted_values:
+        return None
+    n = len(sorted_values)
+    k = max(0, math.ceil((p / 100.0) * n) - 1)
+    return sorted_values[k]
+
+
+def summarize(values: List[float]) -> dict:
+    if not values:
+        return {
+            "count": 0,
+            "avg": None,
+            "min": None,
+            "p50": None,
+            "p90": None,
+            "p95": None,
+        }
+    sorted_vals = sorted(values)
+    return {
+        "count": len(values),
+        "avg": sum(values) / len(values),
+        "min": sorted_vals[0],
+        "p50": _percentile(sorted_vals, 50),
+        "p90": _percentile(sorted_vals, 90),
+        "p95": _percentile(sorted_vals, 95),
+    }