Introduce ELASTIC_APM_CAPTURE_HEADERS

[axw]

Introduce the ELASTIC_APM_CAPTURE_HEADERS config,
which controls whether or not HTTP headers will be captured.
Defaults to true.

Closes #390

[codecov-io]

Codecov Report

Merging #418 into master will increase coverage by 3.43%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master     #418      +/-   ##
==========================================
+ Coverage   80.55%   83.99%   +3.43%     
==========================================
  Files         102      102              
  Lines        6213     6234      +21     
==========================================
+ Hits         5005     5236     +231     
+ Misses        929      925       -4     
+ Partials      279       73     -206

Impacted Files	Coverage Δ
env.go	87.35% <100%> (+6.17%)	⬆️
context.go	94.23% <100%> (+2.15%)	⬆️
error.go	78.8% <100%> (+4.77%)	⬆️
transaction.go	95.37% <100%> (+4.89%)	⬆️
tracer.go	88.61% <88.88%> (+3.38%)	⬆️
modelwriter.go	95.04% <0%> (+0.82%)	⬆️
internal/radix/radix.go	39.92% <0%> (+0.92%)	⬆️
utils.go	80.68% <0%> (+1.13%)	⬆️
internal/sqlscanner/scanner.go	96.82% <0%> (+1.58%)	⬆️
metrics.go	91.66% <0%> (+1.66%)	⬆️
... and 36 more

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