[Backport to 2.x]add more stats: connector count, connector/config index status; fix model count bug

[ylwu-amzn]

Description

Backport #1180 to 2.x

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 #1181 (f3995d8) into 2.x (5c2dc5d) will decrease coverage by 0.10%.
The diff coverage is 53.06%.

@@             Coverage Diff              @@
##                2.x    #1181      +/-   ##
============================================
- Coverage     78.84%   78.74%   -0.10%     
- Complexity     2118     2121       +3     
============================================
  Files           167      167              
  Lines          8641     8676      +35     
  Branches        870      870              
============================================
+ Hits           6813     6832      +19     
- Misses         1432     1450      +18     
+ Partials        396      394       -2     

Flag	Coverage Δ
ml-commons	78.74% <53.06%> (-0.10%)	⬇️

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%)	⬇️
...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 2 files with indirect coverage changes