Tidy up modelwriter

[axw]

Some light refactoring to move "context" model building
logic out of modelwriter and into the Context.build method.

There remains some context model building in modelwriter
(related to setting destination service type) because it
requires additional information outside of "context", i.e.
the span type.

[codecov-io]

Codecov Report

Merging #888 (496de29) into master (697d794) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #888   +/-   ##
=======================================
  Coverage   83.70%   83.70%           
=======================================
  Files         142      142           
  Lines        7959     7960    +1     
=======================================
+ Hits         6662     6663    +1     
  Misses        889      889           
  Partials      408      408           

Impacted Files	Coverage Δ
modelwriter.go	96.62% <ø> (-0.12%)	⬇️
context.go	94.84% <100.00%> (+0.28%)	⬆️
error.go	95.18% <100.00%> (+0.01%)	⬆️
transaction.go	94.02% <100.00%> (ø)

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[apmmachine]

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Build stats

• Build Cause: Pull request #888 opened

• • Start Time: 2021-02-01T03:40:20.571+0000

• Duration: 27 min 24 sec

• Commit: 496de29

Test stats 🧪

Test	Results
Failed	0
Passed	7676
Skipped	188
Total	7864