Optimize flat_object type in a BWC way with one phase processing

[bugmakerrrrrr]

Description

As discussed in #16061, this PR focuses on optimizing flat_object type in a BWC way. I benchmarked this change using noaa workload and set the type of field station as flat_object, the result is as follows.

|                                                        Metric |                     Task |    Baseline |   Contender |     Diff |   Unit |
|--------------------------------------------------------------:|-------------------------:|------------:|------------:|---------:|-------:|
|                                                Min Throughput |                    index |      134517 |      133508 | -1008.98 | docs/s |
|                                               Mean Throughput |                    index |      142195 |      144423 |  2227.85 | docs/s |
|                                             Median Throughput |                    index |      143126 |      146146 |  3020.51 | docs/s |
|                                                Max Throughput |                    index |      150619 |      151649 |   1029.8 | docs/s |
|                                       50th percentile latency |                    index |     206.277 |     210.876 |  4.59914 |     ms |
|                                       90th percentile latency |                    index |     544.938 |     505.005 | -39.9333 |     ms |
|                                       99th percentile latency |                    index |     1225.61 |     1162.57 | -63.0459 |     ms |
|                                     99.9th percentile latency |                    index |     1867.85 |     1568.05 | -299.801 |     ms |
|                                      100th percentile latency |                    index |     1892.49 |     1759.06 |  -133.43 |     ms |
|                                  50th percentile service time |                    index |     206.277 |     210.876 |  4.59914 |     ms |
|                                  90th percentile service time |                    index |     544.938 |     505.005 | -39.9333 |     ms |
|                                  99th percentile service time |                    index |     1225.61 |     1162.57 | -63.0459 |     ms |
|                                99.9th percentile service time |                    index |     1867.85 |     1568.05 | -299.801 |     ms |
|                                 100th percentile service time |                    index |     1892.49 |     1759.06 |  -133.43 |     ms |
|                                                    error rate |                    index |           0 |           0 |        0 |      % |

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 5d37267: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 5dccc88: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 30 lines in your changes missing coverage. Please review.

Project coverage is 71.97%. Comparing base (20536ee) to head (5dccc88).
Report is 150 commits behind head on main.

Files with missing lines	Patch %	Lines
...opensearch/index/mapper/FlatObjectFieldMapper.java	66.66%	20 Missing and 10 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #16297      +/-   ##
============================================
- Coverage     72.00%   71.97%   -0.03%     
+ Complexity    64796    64730      -66     
============================================
  Files          5307     5306       -1     
  Lines        302540   302356     -184     
  Branches      43708    43677      -31     
============================================
- Hits         217853   217630     -223     
- Misses        66801    66853      +52     
+ Partials      17886    17873      -13     

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[bugmakerrrrrr]

@msfroh @kkewwei please take a look when you get a chance

[opensearch-trigger-bot]

This PR is stalled because it has been open for 30 days with no activity.

[opensearch-trigger-bot]

This PR is stalled because it has been open for 30 days with no activity.