Make it possible to conditionally insert attributes

[fippo]

which will be used in #876 where the presence of some
attributes depends on whether they were used in the
original request.

(split from that to make review simpler, no functional changes)

[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.57%. Comparing base (ceccc15) to head (4a5d40b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
agent.go	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #914      +/-   ##
==========================================
+ Coverage   88.52%   88.57%   +0.05%     
==========================================
  Files          45       45              
  Lines        5872     5884      +12     
==========================================
+ Hits         5198     5212      +14     
+ Misses        463      462       -1     
+ Partials      211      210       -1     

Flag	Coverage Δ
go	88.57% <93.54%> (+0.05%)	⬆️

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