fix(sdk-metrics-base): remove metric kind BATCH_OBSERVER

[legendecas]

Which problem is this PR solving?

BatchObserver is no longer a metric since #1709. MetricKind.BATCH_OBSERVER should be removed.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

There are no tests on enum keys.

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

Merging #2540 (95e963a) into main (3045eba) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 95e963a differs from pull request most recent head 6132c08. Consider uploading reports for the commit 6132c08 to get more accurate results

@@            Coverage Diff             @@
##             main    #2540      +/-   ##
==========================================
- Coverage   93.05%   93.05%   -0.01%     
==========================================
  Files         139      139              
  Lines        5172     5171       -1     
  Branches     1110     1110              
==========================================
- Hits         4813     4812       -1     
  Misses        359      359              

Impacted Files	Coverage Δ
...opentelemetry-sdk-metrics-base/src/export/types.ts	100.00% <0.00%> (ø)