fix(clickhouse): cut idle CPU from tiny parts and oversized merge pools

Observability writers flushed every few seconds with MinBatchSize unset,
creating constant small parts and merge load. Enable MinBatchSize with
MaxFlushWait, batch access logs more aggressively, and shrink ClickHouse
background pools for 3c hosts.

internal/db/batchwriter/writer.go

@@ -173,6 +173,7 @@ func (w *Writer[T]) run() {
 	defer ticker.Stop()
 
 	batch := make([]T, 0, w.cfg.MaxBatchSize)
+	var batchStartedAt time.Time
 	flush := func() {
 		if len(batch) == 0 {
 			return
@@ -184,6 +185,7 @@ func (w *Writer[T]) run() {
 			}
 		}
 		batch = batch[:0]
+		batchStartedAt = time.Time{}
 	}
 
 	defer func() {
@@ -197,18 +199,34 @@ func (w *Writer[T]) run() {
 			if !ok {
 				return
 			}
+			if len(batch) == 0 {
+				batchStartedAt = time.Now()
+			}
 			batch = append(batch, item)
 			if len(batch) >= w.cfg.MaxBatchSize {
 				flush()
 			}
 		case <-ticker.C:
-			if len(batch) > 0 && (w.cfg.MinBatchSize == 0 || len(batch) >= w.cfg.MinBatchSize) {
+			if w.shouldFlushOnInterval(len(batch), batchStartedAt, time.Now()) {
 				flush()
 			}
 		}
 	}
 }
 
+func (w *Writer[T]) shouldFlushOnInterval(batchLen int, batchStartedAt time.Time, now time.Time) bool {
+	if batchLen == 0 {
+		return false
+	}
+	if w.cfg.MinBatchSize == 0 || batchLen >= w.cfg.MinBatchSize {
+		return true
+	}
+	if w.cfg.MaxFlushWait <= 0 || batchStartedAt.IsZero() {
+		return false
+	}
+	return !now.Before(batchStartedAt.Add(w.cfg.MaxFlushWait))
+}
+
 func (w *Writer[T]) notifyDrop(item T) {
 	if w.onDrop == nil {
 		return