FIX: missing request index

[chenqi0805]

Description

This fix is a follow up of #3045 . It tackles the generic missing index in bulk request by extracting index value from the first bulk operation.

Bug

2023-07-24T16:51:57,655 [log-pipeline-sink-worker-2-thread-1] WARN  org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink - Document [FailedDlqData{index='test-bulk', indexId='null', status='0', message='Bulk request index cannot be missing', document={log=test log, @timestamp=2023-07-24T16:51:36.582-05:00}}] has failure. DLQ not configured
java.lang.IllegalArgumentException: Bulk request index cannot be missing

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed with a real name per the DCO

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[github-actions]

Unit Test Results

  1 276 files  ±0    1 276 suites  ±0   41m 3s ⏱️ -32s
  5 096 tests +3    5 095 ✔️ +3  1 💤 ±0  0 ❌ ±0 
10 216 runs  +6  10 214 ✔️ +6  2 💤 ±0  0 ❌ ±0 

Results for commit f5c08cb. ± Comparison against base commit 07f00c2.

♻️ This comment has been updated with latest results.

[dlvenable]

Thanks!