remove count from sql_duration_seconds metric

[kcajmagic]

• fix cardinality
• updated changelog

[codecov-io]

Codecov Report

Merging #17 into master will increase coverage by 0.19%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   57.77%   57.97%   +0.19%     
==========================================
  Files          15       15              
  Lines        1035     1035              
==========================================
+ Hits          598      600       +2     
+ Misses        434      433       -1     
+ Partials        3        2       -1

Impacted Files	Coverage Δ
cassandra/executermeasures.go	0% <0%> (ø)	⬆️
cassandra/metrics.go	100% <100%> (ø)	⬆️
batchInserter/batchInserter.go	92.77% <0%> (+2.4%)	⬆️

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