Make tracing dependencies optional

[sagebind]

Fixes #293.

[codecov[bot]]

Codecov Report

Base: 52.47% // Head: 52.49% // Increases project coverage by +0.02% :tada:

Coverage data is based on head (a25239c) compared to base (6917903). Patch coverage: 48.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
+ Coverage   52.47%   52.49%   +0.02%     
==========================================
  Files          53       53              
  Lines        5795     5781      -14     
==========================================
- Hits         3041     3035       -6     
+ Misses       2754     2746       -8     

Impacted Files	Coverage Δ
src/agent/selector.rs	75.94% <0.00%> (ø)
src/cookies/interceptor.rs	73.91% <0.00%> (ø)
src/redirect.rs	78.35% <0.00%> (ø)
src/text.rs	81.39% <0.00%> (ø)
src/trailer.rs	58.92% <0.00%> (ø)
src/cookies/psl/mod.rs	95.08% <33.33%> (ø)
src/handler.rs	81.04% <47.61%> (+0.22%)	:arrow_up:
src/client.rs	79.33% <50.00%> (-2.96%)	:arrow_down:
src/agent/mod.rs	77.77% <58.06%> (ø)
src/cookies/jar.rs	83.57% <75.00%> (ø)
... and 6 more

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