avoid loading application log4j plugins

[SylvainJuge]

Found while testing with an application that is using log4j2.

Shaded log4j2 from agent tries to load application plugins, this is due to having hard-coded references to Log4j2Plugins.dat within META-INF folder (which is not covered by relocation patterns)

Classes that trigger this issue can be found using the following one-liner command within an expanded copy of the agent jar.

find . -name '*.class' | xargs grep 'META-INF/org/' -ali 

We only have two of them

./co/elastic/apm/agent/shaded/apache/logging/log4j/core/config/plugins/processor/PluginProcessor.class
./co/elastic/apm/agent/shaded/apache/logging/log4j/core/config/plugins/util/PluginRegistry.class

[apmmachine]

💚 Build Succeeded

Expand to view the summary

Build stats

• Build Cause: [Pull request #1214 updated]

• Start Time: 2020-06-05T07:34:38.241+0000

• Duration: 37 min 29 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1332
Skipped	12
Total	1344

[codecov-commenter]

Codecov Report

Merging #1214 into master will increase coverage by 0.65%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #1214      +/-   ##
============================================
+ Coverage     59.24%   59.90%   +0.65%     
- Complexity       87       94       +7     
============================================
  Files           339      344       +5     
  Lines         16026    16680     +654     
  Branches       2248     2384     +136     
============================================
+ Hits           9495     9992     +497     
- Misses         5881     6006     +125     
- Partials        650      682      +32     

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/co/elastic/apm/agent/util/IOUtils.java	56.88% <50.00%> (-1.22%)	0.00 <0.00> (ø)
...lastic/apm/agent/servlet/AsyncInstrumentation.java	63.04% <0.00%> (-1.41%)	0.00% <0.00%> (ø%)
...astic/apm/agent/impl/transaction/AbstractSpan.java	75.18% <0.00%> (-0.26%)	0.00% <0.00%> (ø%)
...lastic/apm/agent/objectpool/ObjectPoolFactory.java	100.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...lconnection/HttpsUrlConnectionInstrumentation.java	80.00% <0.00%> (ø)	0.00% <0.00%> (?%)
...t/grails/GrailsTransactionNameInstrumentation.java	40.47% <0.00%> (ø)	7.00% <0.00%> (?%)
...main/java/co/elastic/apm/agent/util/CallDepth.java	0.00% <0.00%> (ø)	0.00% <0.00%> (?%)
...t/RunnableCallableForkJoinTaskInstrumentation.java	69.23% <0.00%> (ø)	0.00% <0.00%> (?%)
...o/elastic/apm/agent/concurrent/JavaConcurrent.java	71.87% <0.00%> (ø)	0.00% <0.00%> (?%)
... and 7 more

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