Avoid choices evaluation on relational fields getting field metadata

[santiavenda2]

On relation fields line 131 is getting all the related objects.

[codecov-io]

Codecov Report

Merging #336 into develop will decrease coverage by 18.79%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop     #336      +/-   ##
===========================================
- Coverage    91.68%   72.88%   -18.8%     
===========================================
  Files           50       50              
  Lines         2368     5676    +3308     
===========================================
+ Hits          2171     4137    +1966     
- Misses         197     1539    +1342

Impacted Files	Coverage Δ
rest_framework_json_api/metadata.py	38.52% <0%> (+10.19%)	⬆️
rest_framework_json_api/pagination.py	50.57% <0%> (-41.93%)	⬇️
rest_framework_json_api/parsers.py	54.04% <0%> (-37.63%)	⬇️
example/models.py	59.65% <0%> (-35.17%)	⬇️
rest_framework_json_api/renderers.py	57.94% <0%> (-28.31%)	⬇️
example/settings/test.py	89.12% <0%> (-10.88%)	⬇️
rest_framework_json_api/mixins.py	81.94% <0%> (-5.56%)	⬇️
rest_framework_json_api/exceptions.py	83.42% <0%> (-1.19%)	⬇️
example/migrations/0001_initial.py	100% <0%> (ø)	⬆️
rest_framework_json_api/relations.py	84.86% <0%> (+1.78%)	⬆️
... and 1 more

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[mblayman]

Looks good to me. 👍