Fix attach external config

[SylvainJuge]

What does this PR do?

When using remote attach, if there is any external configuration provided through config_file option, this external configuration file is ignored.

The expected behavior here should be to:

• load the external configuration first
• allow to override configuration items from the external configuration file with entries in the --config provided through CLI.

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix

[felixbarny]

just a minor suggestion, otherwise LGTM

[felixbarny]

use try-with-resources

[SylvainJuge]

Thanks, I always forget that one !

[ghost]

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Build stats

• Build Cause: [Pull request #1469 updated]

• Start Time: 2020-11-03T08:44:47.639+0000

• Duration: 46 min 36 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1617
Skipped	12
Total	1629

[SylvainJuge]

I have updated documentation for the most common case (remote attach with CLI), but in practice it will also make it possible to use elasticapm.properties either in classpath or next to agent jar to include a config_file entry, in which case we might not properly define/enforce ordering and priority.

I think it's unlikely enough that we can wait until it becomes an issue before dealing with that.

[codecov-io]

Codecov Report

Merging #1469 into master will increase coverage by 3.74%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1469      +/-   ##
============================================
+ Coverage     59.73%   63.48%   +3.74%     
- Complexity       91     3297    +3206     
============================================
  Files           391      344      -47     
  Lines         17528    16055    -1473     
  Branches       2408     2196     -212     
============================================
- Hits          10470    10192     -278     
+ Misses         6340     5173    -1167     
+ Partials        718      690      -28     

Impacted Files	Coverage Δ	Complexity Δ
...acing/impl/ExternalSpanContextInstrumentation.java
...t/kafka/helper/KafkaInstrumentationHelperImpl.java
...ent/kafka/KafkaProducerHeadersInstrumentation.java
...afka/ConsumerRecordsRecordListInstrumentation.java
...ic/apm/opentracing/ExternalProcessSpanContext.java
...agent/opentracing/impl/ApmSpanInstrumentation.java
...agent/kafka/helper/ConsumerRecordsListWrapper.java
...t/opentracing/impl/SpanContextInstrumentation.java
...astic/apm/opentracing/TraceContextSpanContext.java
...ent/opentracing/impl/OpenTracingTextMapBridge.java
... and 38 more

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[SylvainJuge]

I've just realized that the initial implemented ordering wasn't consistent with what was documented, thus I've made config_file option have higher priority than configuration provided through runtime attach parameters.