[exporterhelper] Fix persistent queue size backup on reads (#9740)

**Description:** 

Persistent queue size backup on reads should depend on readIndex, not
writeIndex.

Author: carsonip

.chloggen/carsonip_persistent-queue-size-backup.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: exporterhelper
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix persistent queue size backup on reads.
+
+# One or more tracking issues or pull requests related to the change
+issues: [9740]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

exporter/internal/queue/persistent_queue.go

@@ -341,7 +341,7 @@ func (pq *persistentQueue[T]) getNextItem(ctx context.Context) (T, func(error),
 
 	// Back up the queue size to storage on every 10 reads. The stored value is used to recover the queue size
 	// in case if the collector is killed. The recovered queue size is allowed to be inaccurate.
-	if (pq.writeIndex % 10) == 0 {
+	if (pq.readIndex % 10) == 0 {
 		if qsErr := pq.backupQueueSize(ctx); qsErr != nil {
 			pq.logger.Error("Error writing queue size to storage", zap.Error(err))
 		}