Use synchronized lists in MockReporter

[felixbarny]

attempt to fix #1178

[codecov-io]

Codecov Report

Merging #1179 into master will decrease coverage by 4.37%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1179      +/-   ##
============================================
- Coverage     64.05%   59.67%   -4.38%     
+ Complexity     2794       83    -2711     
============================================
  Files           282      328      +46     
  Lines         13609    15020    +1411     
  Branches       1874     2089     +215     
============================================
+ Hits           8717     8963     +246     
- Misses         4305     5447    +1142     
- Partials        587      610      +23     

Impacted Files	Coverage Δ	Complexity Δ
...nt/configuration/converter/ListValueConverter.java	43.47% <0.00%> (-39.86%)	0.00% <0.00%> (-4.00%)
...ration/source/PropertyFileConfigurationSource.java	30.26% <0.00%> (-35.46%)	0.00% <0.00%> (-10.00%)
...tion/source/SystemPropertyConfigurationSource.java	23.07% <0.00%> (-26.93%)	0.00% <0.00%> (-3.00%)
...c/main/java/co/elastic/apm/agent/util/IOUtils.java	44.08% <0.00%> (-19.98%)	0.00% <0.00%> (-10.00%)
.../agent/hibernate/search/HibernateSearchHelper.java	0.00% <0.00%> (ø)	0.00% <0.00%> (?%)
...m/agent/kafka/BaseKafkaHeadersInstrumentation.java	0.00% <0.00%> (ø)	0.00% <0.00%> (?%)
...Mule4OverrideClassLoaderLookupInstrumentation.java	0.00% <0.00%> (ø)	0.00% <0.00%> (?%)
...pentracing/impl/ApmSpanBuilderInstrumentation.java	0.00% <0.00%> (ø)	0.00% <0.00%> (?%)
...ava/co/elastic/apm/opentracing/ElasticApmTags.java	0.00% <0.00%> (ø)	0.00% <0.00%> (?%)
...gent/opentracing/impl/ApmScopeInstrumentation.java	0.00% <0.00%> (ø)	0.00% <0.00%> (?%)
... and 42 more

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💚 Build Succeeded

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Build stats

• Build Cause: [Pull request Use synchronized lists in MockReporter #1179 opened]

• Start Time: 2020-05-12T07:38:57.165+0000

• Duration: 38 min 37 sec (2256824)

• Commit: 3466965

Test stats 🧪

Test	Results
Failed	0
Passed	1263
Skipped	12
Total	1275