Error handling of Servlet Async API

[eyalkoren]

Status code and error reporting
Pool ApmAsynListener objects
Extend integration tests to capturing errors and JSP transactions

[eyalkoren]

Jenkins test this please

[codecov-io]

Codecov Report

Merging #399 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #399      +/-   ##
============================================
- Coverage     72.26%   72.24%   -0.02%     
+ Complexity     1339     1315      -24     
============================================
  Files           145      143       -2     
  Lines          5257     5063     -194     
  Branches        550      518      -32     
============================================
- Hits           3799     3658     -141     
+ Misses         1222     1169      -53     
  Partials        236      236

Impacted Files	Coverage Δ	Complexity Δ
...lastic/apm/agent/servlet/AsyncInstrumentation.java	68.57% <ø> (ø)	3 <0> (ø)	⬇️
...tic/apm/agent/servlet/helper/ApmAsyncListener.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...t/servlet/helper/AsyncContextAdviceHelperImpl.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...astic/apm/agent/impl/transaction/TraceContext.java	92.21% <0%> (-0.52%)	56% <0%> (-11%)
...java/co/elastic/apm/agent/bci/ElasticApmAgent.java	73.04% <0%> (-0.33%)	22% <0%> (-2%)
...webmvc/DispatcherServletRenderInstrumentation.java
...bci/bytebuddy/FailSafeDeclaredMethodsCompiler.java
.../agent/plugin/api/AbstractSpanInstrumentation.java	53.19% <0%> (+1.15%)	3% <0%> (-3%)	⬇️
...agent/plugin/api/ElasticApmApiInstrumentation.java	52% <0%> (+12%)	3% <0%> (ø)	⬇️

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[felixbarny]

The number of required ApmAsyncListener instances is independent of the size of the disruptor. It's probably dependant on the number of container threads. The default for Tomcat is 200 IIRC. I'm not sure whether there is an extra thread pool for the handling of async requests but I suppose there is not.

Maybe hard-code to 256?

[eyalkoren]

You are right it is not the right configuration to rely on. We can start with hard code and see if it bites us.