[api] Refactor metrics API

[frankfliu]

Change-Id: I271ec2b5b89034d837949eee4829016321550e34

Description

Brief description of what this PR is about

• If this change is a backward incompatible change, why must this change be made?
• Interesting edge cases to note here

[codecov-commenter]

Codecov Report

Merging #1613 (73edd2a) into master (bb5073f) will decrease coverage by 1.21%.
The diff coverage is 61.87%.

@@             Coverage Diff              @@
##             master    #1613      +/-   ##
============================================
- Coverage     72.08%   70.87%   -1.22%     
- Complexity     5126     5420     +294     
============================================
  Files           473      506      +33     
  Lines         21970    23712    +1742     
  Branches       2351     2582     +231     
============================================
+ Hits          15838    16805     +967     
- Misses         4925     5621     +696     
- Partials       1207     1286      +79     

Impacted Files	Coverage Δ
api/src/main/java/ai/djl/modality/cv/Image.java	69.23% <ø> (-4.11%)	⬇️
...i/djl/modality/cv/translator/BigGANTranslator.java	21.42% <ø> (-5.24%)	⬇️
...odality/cv/translator/BigGANTranslatorFactory.java	33.33% <0.00%> (+8.33%)	⬆️
...nslator/InstanceSegmentationTranslatorFactory.java	14.28% <0.00%> (-3.90%)	⬇️
.../modality/cv/translator/YoloTranslatorFactory.java	8.33% <0.00%> (-1.67%)	⬇️
...i/djl/modality/cv/translator/YoloV5Translator.java	5.69% <0.00%> (ø)
...odality/cv/translator/YoloV5TranslatorFactory.java	8.33% <0.00%> (-1.67%)	⬇️
...pi/src/main/java/ai/djl/ndarray/BytesSupplier.java	54.54% <0.00%> (-12.13%)	⬇️
...ain/java/ai/djl/ndarray/index/dim/NDIndexPick.java	100.00% <ø> (ø)
api/src/main/java/ai/djl/nn/Blocks.java	75.00% <0.00%> (-25.00%)	⬇️
... and 239 more

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