module/apmot: follows-from is not supported

[axw]

Bring the code in line with the docs, and do not support follows-from.

Fixes #413

[codecov-io]

Codecov Report

Merging #414 into master will increase coverage by 3.24%.
The diff coverage is 86.66%.

@@            Coverage Diff            @@
##           master    #414      +/-   ##
=========================================
+ Coverage   80.66%   83.9%   +3.24%     
=========================================
  Files         102     102              
  Lines        6148    6151       +3     
=========================================
+ Hits         4959    5161     +202     
- Misses        915     919       +4     
+ Partials      274      71     -203

Impacted Files	Coverage Δ
module/apmot/context.go	80% <86.66%> (-6.96%)	⬇️
transport/transporttest/recorder.go	82.19% <0%> (ø)	⬆️
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%)	⬆️
context.go	94.05% <0%> (+1.98%)	⬆️
internal/apmhttputil/url.go	97.87% <0%> (+2.12%)	⬆️
module/apmsql/stmt.go	63.82% <0%> (+2.12%)	⬆️
... and 32 more

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