[Backport 2.x] Add graph creation stats to KNNStats API

[ryanbogan]

Description

Backport of #1141

Issues Resolved

Check List

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

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[codecov]

Codecov Report

Merging #1191 (a4d8bd2) into 2.x (39836c6) will increase coverage by 0.14%.
The diff coverage is 92.38%.

@@             Coverage Diff              @@
##                2.x    #1191      +/-   ##
============================================
+ Coverage     84.98%   85.12%   +0.14%     
- Complexity     1168     1187      +19     
============================================
  Files           154      155       +1     
  Lines          4781     4881     +100     
  Branches        442      450       +8     
============================================
+ Hits           4063     4155      +92     
- Misses          520      527       +7     
- Partials        198      199       +1     

Files	Coverage Δ
...index/codec/KNN80Codec/KNN80DocValuesConsumer.java	91.03% <100.00%> (+2.33%)	⬆️
...n/index/codec/util/KNNVectorSerializerFactory.java	96.29% <ø> (ø)
...org/opensearch/knn/plugin/stats/KNNGraphValue.java	100.00% <100.00%> (ø)
...java/org/opensearch/knn/plugin/stats/KNNStats.java	100.00% <100.00%> (ø)
...ava/org/opensearch/knn/plugin/stats/StatNames.java	100.00% <100.00%> (ø)
.../opensearch/knn/index/codec/util/KNNCodecUtil.java	77.50% <66.66%> (-16.95%)	⬇️