fix: URL encode explicit routing values

[alevenberg]

Part of #12232

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This change is 

[alevenberg]

I ran GENERATE_GOLDEN_ONLY=true ci/cloudbuild/build.sh -t generate-libraries-pr to generate the generated file change and ci/cloudbuild/build.sh -t asan-pr to verify the change builds

I'm not sure what unit tests I should change for this change

[dbolduc]

• GENERATE_GOLDEN_ONLY is good for developing, but we should remove it and rerun generate-libraries-pr before sending the PR.
• I would add a new unit test in golden_kitchen_sink_metadata_decorator_test.cc. Look at the ExplicitRouting* tests for inspiration.
• We should probably make identical changes to the REST metadata decorator

[alevenberg]

I did the first two.

We should probably make identical changes to the REST metadata decorator

Maybe in a 2nd PR?

[codecov]

Codecov Report

Patch coverage: 88.46% and project coverage change: -0.01% ⚠️

Comparison is base (af7e22c) 93.62% compared to head (f47c1e9) 93.61%.
Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12493      +/-   ##
==========================================
- Coverage   93.62%   93.61%   -0.01%     
==========================================
  Files        2039     2039              
  Lines      180676   180697      +21     
==========================================
+ Hits       169153   169168      +15     
- Misses      11523    11529       +6     

Files Changed	Coverage Δ
generator/internal/metadata_decorator_generator.cc	34.54% <0.00%> (ø)
...internal/golden_kitchen_sink_metadata_decorator.cc	84.31% <50.00%> (ø)
...sts/golden_kitchen_sink_metadata_decorator_test.cc	99.64% <100.00%> (+0.02%)	⬆️

... and 5 files with indirect coverage changes

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[dbolduc]

Maybe in a 2nd PR?

SGTM