add more stats: connector count, connector/config index status; fix model count bug

[ylwu-amzn]

Description

Add more stats:

• connector count
• connector index status
• config index status

Fix model count bug: local model may have multiple chunks, the model count number should not include model chunk.

Also fine tune some error message

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

Merging #1180 (91eeae3) into 2.9 (82bddb1) will decrease coverage by 0.18%.
Report is 1 commits behind head on 2.9.
The diff coverage is 57.89%.

@@             Coverage Diff              @@
##                2.9    #1180      +/-   ##
============================================
- Coverage     78.92%   78.75%   -0.18%     
- Complexity     2119     2121       +2     
============================================
  Files           167      167              
  Lines          8633     8676      +43     
  Branches        869      870       +1     
============================================
+ Hits           6814     6833      +19     
- Misses         1423     1448      +25     
+ Partials        396      395       -1     

Flag	Coverage Δ
ml-commons	78.75% <57.89%> (-0.18%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...va/org/opensearch/ml/engine/utils/ScriptUtils.java	76.47% <ø> (ø)
...a/org/opensearch/ml/model/MLModelGroupManager.java	84.48% <ø> (ø)
.../main/java/org/opensearch/ml/utils/IndexUtils.java	41.17% <0.00%> (-22.46%)	⬇️
.../opensearch/ml/engine/encryptor/EncryptorImpl.java	91.04% <66.66%> (ø)
...java/org/opensearch/ml/rest/RestMLStatsAction.java	91.66% <78.94%> (-3.24%)	⬇️
...ch/ml/engine/algorithms/remote/ConnectorUtils.java	70.83% <87.50%> (+2.08%)	⬆️
...rg/opensearch/ml/plugin/MachineLearningPlugin.java	99.04% <100.00%> (+0.03%)	⬆️
...va/org/opensearch/ml/stats/MLClusterLevelStat.java	80.00% <100.00%> (+8.57%)	⬆️

... and 3 files with indirect coverage changes

[dhrubo-os]

Should we update to Please provide a different model group name. We can see people started having confusion: #1179

about: \"model_group_id\": \"lMPmr4kB4eSCtCCDmCDm\" : Is this any static group id we are adding?

[ylwu-amzn]

Fixed hard coded model id .
For Please provide a different model group name , that looks also confusing as user just provide model name in the request body, not model group name,

POST _plugins/_ml/models/_upload
{
  "name": "huggingface/sentence-transformers/all-MiniLM-L12-v2",
  "version": "1.0.1",
  "model_format": "TORCH_SCRIPT"
}

[Zhangxunmt]

We may add more granularity stats like connector/model count for sageMaker, etc, if necessary in the future.