[exporterhelper] Improve retry sender log message clarity

[Vanshul97]

Summary

Improves the retry sender log message to reduce user confusion about transient export failures.

Fixes #13943

Problem

The current log message:

INFO "Exporting failed. Will retry the request after interval." error="rpc error: code = DeadlineExceeded desc = context deadline exceeded" interval="9.64s"

This causes concern because:

• "Exporting failed" sounds alarming — users cannot tell if data is being lost
• No indication that the error is transient and will be automatically retried
• No retry count to gauge how long the issue has persisted

Fix

Changed the log message to:

INFO "Transient error, will retry the request after interval." error="..." interval="9.64s" retry_num=3

Changes:

1. Message: "Exporting failed. Will retry..." → "Transient error, will retry..." — immediately signals this is a recoverable, transient failure
2. Added retry_num: Shows which retry attempt this is, helping operators gauge severity
3. Moved retryNum++ before logging: So the logged retry number is 1-based (first retry = 1)
4. Span event updated: Same improvements applied to the trace span event, plus retry_num attribute added

Test Plan

• TestRetrySenderWithContextTimeout updated to expect new message
• All retry sender tests pass with -race