Use temp properties file instead of providing agent arguments

[felixbarny]

closes #864

Checklist

• Implement code
• Add tests
• Update documentation
• Update CHANGELOG.md
• Update supported-technologies.asciidoc
• Added an API method or config option? Document in which version this will be introduced.

[codecov-io]

Codecov Report

Merging #865 into master will decrease coverage by 0.6%.
The diff coverage is 56.25%.

@@             Coverage Diff              @@
##             master     #865      +/-   ##
============================================
- Coverage     64.83%   64.22%   -0.61%     
- Complexity       84       85       +1     
============================================
  Files           230      230              
  Lines          9449     9358      -91     
  Branches       1245     1221      -24     
============================================
- Hits           6126     6010     -116     
- Misses         2945     2963      +18     
- Partials        378      385       +7

Impacted Files	Coverage Δ	Complexity Δ
...nfiguration/AgentArgumentsConfigurationSource.java	92.3% <ø> (ø)	0 <0> (ø)	⬇️
...ration/source/PropertyFileConfigurationSource.java	30.26% <ø> (ø)	0 <0> (ø)	⬇️
...java/co/elastic/apm/attach/ElasticApmAttacher.java	20.54% <44.44%> (+8.26%)	4 <1> (+1)	⬆️
...lastic/apm/agent/impl/ElasticApmTracerBuilder.java	79.01% <71.42%> (-0.4%)	0 <0> (ø)
.../apm/agent/report/serialize/DslJsonSerializer.java	86.08% <0%> (-3.63%)	0% <0%> (ø)

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[hcf]

Could it be possible to use system properties instead as suggested in the temporary fix for my problem?

configuration.forEach((key, value) -> System.setProperty(String.format("elastic.apm.%s", key), value));

Not sure what is best for concurrency, conflicts, etc.

[felixbarny]

That wouldn't work with the external attachment (https://www.elastic.co/guide/en/apm/agent/java/current/setup-attach-cli.html). Do you see an issue with the proposed approach?

[hcf]

The only issue I can think about is what happens if we start multiple JVM on the same machine at the same time?

[felixbarny]

Then each JVM will get their own temporary file (with a random name) which gets deleted after the attachment for that JVM is completed.