Adding context.message.age.ms field

[eyalkoren]

Implementing the latest requirement in elastic/apm#143 for the addition of a context.message.age.ms field.

Checklist

• Implement code
• Add tests
• Update CHANGELOG.asciidoc

[codecov-io]

Codecov Report

Merging #970 into master will decrease coverage by 0.32%.
The diff coverage is 46.42%.

@@             Coverage Diff              @@
##             master     #970      +/-   ##
============================================
- Coverage      63.5%   63.18%   -0.33%     
  Complexity       85       85              
============================================
  Files           245      245              
  Lines         10172    10083      -89     
  Branches       1351     1333      -18     
============================================
- Hits           6460     6371      -89     
- Misses         3326     3329       +3     
+ Partials        386      383       -3

Impacted Files	Coverage Δ	Complexity Δ
...lastic/apm/agent/jms/JmsInstrumentationHelper.java	100% <ø> (ø)	0 <0> (ø)	⬇️
...ic/apm/agent/jms/JmsInstrumentationHelperImpl.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...m/agent/jms/JmsMessageConsumerInstrumentation.java	15.45% <0%> (-0.29%)	0 <0> (ø)
...m/agent/jms/JmsMessageListenerInstrumentation.java	20.4% <0%> (-1.34%)	0 <0> (ø)
...ava/co/elastic/apm/agent/impl/context/Message.java	96.96% <100%> (+0.67%)	0 <0> (ø)	⬇️
.../apm/agent/report/serialize/DslJsonSerializer.java	90.76% <100%> (+0.08%)	0 <0> (ø)	⬇️
...astic/apm/agent/impl/transaction/AbstractSpan.java	80.15% <0%> (-3.44%)	0% <0%> (ø)
...va/co/elastic/apm/agent/impl/transaction/Span.java	77.52% <0%> (-2.04%)	0% <0%> (ø)

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[SylvainJuge]

LGTM