[elasticsearch] Add tests

[ricoberger]

Add tests for Elasticsearch plugin to improve code coverage.

• I added a CHANGELOG entry for this change.
• I adjusted the corresponding documentation for this change.
• I adjusted the values.yaml file and the documentation for all values.

[codecov-commenter]

Codecov Report

Merging #261 (c306ed5) into main (4c8d592) will increase coverage by 1.84%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##             main     #261      +/-   ##
==========================================
+ Coverage   14.36%   16.20%   +1.84%     
==========================================
  Files          89       89              
  Lines        7534     7533       -1     
==========================================
+ Hits         1082     1221     +139     
+ Misses       6435     6295     -140     
  Partials       17       17              

Impacted Files	Coverage Δ
cmd/kobs/plugins/plugins.go	0.00% <0.00%> (ø)
plugins/elasticsearch/elasticsearch.go	100.00% <100.00%> (+100.00%)	⬆️
plugins/elasticsearch/pkg/instance/instance.go	100.00% <100.00%> (+100.00%)	⬆️

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