Add dedicated feature flag for breakdown_metrics

[felixbarny]

• Implement code
• Add tests
• Update documentation

[felixbarny]

run the tests

[codecov-io]

Codecov Report

Merging #743 into master will decrease coverage by 0.51%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #743      +/-   ##
============================================
- Coverage     64.79%   64.28%   -0.52%     
+ Complexity      169       68     -101     
============================================
  Files           209      209              
  Lines          8925     8705     -220     
  Branches       1147     1101      -46     
============================================
- Hits           5783     5596     -187     
+ Misses         2788     2767      -21     
+ Partials        354      342      -12

Impacted Files	Coverage Δ	Complexity Δ
...apm/agent/impl/transaction/TraceContextHolder.java	53.19% <0%> (-5.15%)	0% <0%> (-2%)
...nt/configuration/ApmServerConfigurationSource.java	90% <0%> (-2.86%)	0% <0%> (ø)
...va/co/elastic/apm/agent/impl/ElasticApmTracer.java	74.05% <0%> (-1.7%)	0% <0%> (-33%)
...astic/apm/agent/impl/transaction/TraceContext.java	90.25% <0%> (-1.03%)	0% <0%> (-19%)
...tic/apm/agent/configuration/CoreConfiguration.java	96.46% <0%> (-0.49%)	0% <0%> (ø)
...lastic/apm/agent/impl/transaction/Transaction.java	84.29% <0%> (-0.41%)	0% <0%> (-25%)
...lastic/apm/agent/report/ReporterConfiguration.java	100% <0%> (ø)	0% <0%> (ø)	⬇️
...agent/impl/stacktrace/StacktraceConfiguration.java	100% <0%> (ø)	0% <0%> (-3%)	⬇️
...ic/apm/agent/slf4j/Slf4JMdcActivationListener.java	89.13% <0%> (+0.24%)	0% <0%> (-7%)	⬇️
...lastic/apm/agent/logging/LoggingConfiguration.java	92.3% <0%> (+0.74%)	0% <0%> (-12%)	⬇️
... and 1 more

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