do not split batches in Log Emitter

This changes the Log Emitter to run the `consumerFunc`
on the whole batch, instead of splitting the batch into individual entries
and calling `consumerFunc` on each of them.

This doesn't change much while the Log Emitter has its own `batch` buffer,
but if we remove the `batch` buffer (see #35456),
this should prevent the performance drop described in #35454.

pkg/stanza/operator/helper/emitter.go

@@ -104,9 +104,9 @@ func (e *LogEmitter) Process(ctx context.Context, ent *entry.Entry) error {
 }
 
 // ProcessBatch emits the entries to the consumerFunc
-func (e *LogEmitter) ProcessBatch(ctx context.Context, entries []entry.Entry) error {
-	for _, entry := range entries {
-		e.Process(ctx, &entry)
+func (e *LogEmitter) ProcessBatch(ctx context.Context, entries []*entry.Entry) error {
+	if oldBatch := e.appendEntries(entries); len(oldBatch) > 0 {
+		e.consumerFunc(ctx, oldBatch)
 	}
 
 	return nil
@@ -128,6 +128,22 @@ func (e *LogEmitter) appendEntry(ent *entry.Entry) []*entry.Entry {
 	return nil
 }
 
+// appendEntries appends the entries to the current batch. If maxBatchSize is reached, a new batch will be made, and the old batch
+// (which should be flushed) will be returned
+func (e *LogEmitter) appendEntries(entries []*entry.Entry) []*entry.Entry {
+	e.batchMux.Lock()
+	defer e.batchMux.Unlock()
+
+	e.batch = append(e.batch, entries...)
+	if uint(len(e.batch)) >= e.maxBatchSize {
+		var oldBatch []*entry.Entry
+		oldBatch, e.batch = e.batch, make([]*entry.Entry, 0, e.maxBatchSize)
+		return oldBatch
+	}
+
+	return nil
+}
+
 // flusher flushes the current batch every flush interval. Intended to be run as a goroutine
 func (e *LogEmitter) flusher() {
 	defer e.wg.Done()