chore: fixing coverage for karma using istanbul

[obecny]

Which problem is this PR solving?

1. Fixes Fix karma coverage for browsers tests #455

Short description of the changes

Fixing coverage tests with karma

[codecov-io]

Codecov Report

Merging #466 into master will increase coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #466      +/-   ##
=========================================
+ Coverage   95.06%   95.3%   +0.24%     
=========================================
  Files         132     136       +4     
  Lines        6782    6926     +144     
  Branches      568     597      +29     
=========================================
+ Hits         6447    6601     +154     
+ Misses        335     325      -10

Impacted Files	Coverage Δ
...es/opentelemetry-node/src/instrumentation/utils.ts	90.47% <0%> (-5.53%)	⬇️
...ages/opentelemetry-plugin-http/test/utils/utils.ts	57.14% <0%> (-2.86%)	⬇️
packages/opentelemetry-metrics/src/Utils.ts	100% <0%> (ø)	⬆️
...try-node/test/instrumentation/PluginLoader.test.ts	100% <0%> (ø)	⬆️
...opentelemetry-plugin-http/test/utils/assertSpan.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-metrics/src/types.ts
...es/opentelemetry-plugin-document-load/src/utils.ts	100% <0%> (ø)
...y-plugin-document-load/src/enums/AttributeNames.ts	100% <0%> (ø)
...ackages/opentelemetry-web/src/StackScopeManager.ts	100% <0%> (ø)
...-document-load/src/enums/PerformanceTimingNames.ts	100% <0%> (ø)
... and 6 more