refactor: streamline usage reporting by consolidating record publishing logic

- Introduced a new method `buildRecord` in `usageReporter` to encapsulate record creation, improving code readability and maintainability.
- Added latency tracking to usage records, ensuring accurate reporting of request latencies.
- Updated tests to validate the inclusion of latency in usage records and ensure proper functionality of the new reporting structure.

internal/usage/logger_plugin_test.go

@@ -0,0 +1,96 @@
+package usage
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	coreusage "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/usage"
+)
+
+func TestRequestStatisticsRecordIncludesLatency(t *testing.T) {
+	stats := NewRequestStatistics()
+	stats.Record(context.Background(), coreusage.Record{
+		APIKey:      "test-key",
+		Model:       "gpt-5.4",
+		RequestedAt: time.Date(2026, 3, 20, 12, 0, 0, 0, time.UTC),
+		Latency:     1500 * time.Millisecond,
+		Detail: coreusage.Detail{
+			InputTokens:  10,
+			OutputTokens: 20,
+			TotalTokens:  30,
+		},
+	})
+
+	snapshot := stats.Snapshot()
+	details := snapshot.APIs["test-key"].Models["gpt-5.4"].Details
+	if len(details) != 1 {
+		t.Fatalf("details len = %d, want 1", len(details))
+	}
+	if details[0].LatencyMs != 1500 {
+		t.Fatalf("latency_ms = %d, want 1500", details[0].LatencyMs)
+	}
+}
+
+func TestRequestStatisticsMergeSnapshotDedupIgnoresLatency(t *testing.T) {
+	stats := NewRequestStatistics()
+	timestamp := time.Date(2026, 3, 20, 12, 0, 0, 0, time.UTC)
+	first := StatisticsSnapshot{
+		APIs: map[string]APISnapshot{
+			"test-key": {
+				Models: map[string]ModelSnapshot{
+					"gpt-5.4": {
+						Details: []RequestDetail{{
+							Timestamp: timestamp,
+							LatencyMs: 0,
+							Source:    "user@example.com",
+							AuthIndex: "0",
+							Tokens: TokenStats{
+								InputTokens:  10,
+								OutputTokens: 20,
+								TotalTokens:  30,
+							},
+						}},
+					},
+				},
+			},
+		},
+	}
+	second := StatisticsSnapshot{
+		APIs: map[string]APISnapshot{
+			"test-key": {
+				Models: map[string]ModelSnapshot{
+					"gpt-5.4": {
+						Details: []RequestDetail{{
+							Timestamp: timestamp,
+							LatencyMs: 2500,
+							Source:    "user@example.com",
+							AuthIndex: "0",
+							Tokens: TokenStats{
+								InputTokens:  10,
+								OutputTokens: 20,
+								TotalTokens:  30,
+							},
+						}},
+					},
+				},
+			},
+		},
+	}
+
+	result := stats.MergeSnapshot(first)
+	if result.Added != 1 || result.Skipped != 0 {
+		t.Fatalf("first merge = %+v, want added=1 skipped=0", result)
+	}
+
+	result = stats.MergeSnapshot(second)
+	if result.Added != 0 || result.Skipped != 1 {
+		t.Fatalf("second merge = %+v, want added=0 skipped=1", result)
+	}
+
+	snapshot := stats.Snapshot()
+	details := snapshot.APIs["test-key"].Models["gpt-5.4"].Details
+	if len(details) != 1 {
+		t.Fatalf("details len = %d, want 1", len(details))
+	}
+}