feat(generator): support explicit routing headers

[dbolduc]

Fixes #9121

The first and last commits are interesting. The middle two are not.

We do not pass our variables through service_method_vars. It is easier to work with the data directly.

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[codecov]

Codecov Report

Merging #9368 (a6af996) into main (ec741cb) will decrease coverage by 0.08%.
The diff coverage is 55.91%.

@@            Coverage Diff             @@
##             main    #9368      +/-   ##
==========================================
- Coverage   94.68%   94.60%   -0.09%     
==========================================
  Files        1480     1480              
  Lines      135483   135757     +274     
==========================================
+ Hits       128285   128431     +146     
- Misses       7198     7326     +128     

Impacted Files	Coverage Δ
...gration_tests/golden/golden_kitchen_sink_client.cc	80.00% <0.00%> (-8.32%)	⬇️
...egration_tests/golden/golden_kitchen_sink_client.h	100.00% <ø> (ø)
...ion_tests/golden/golden_kitchen_sink_connection.cc	41.02% <0.00%> (-4.69%)	⬇️
...tion_tests/golden/golden_kitchen_sink_connection.h	100.00% <ø> (ø)
...lden_kitchen_sink_connection_idempotency_policy.cc	72.72% <0.00%> (-16.17%)	⬇️
...tegration_tests/golden/golden_thing_admin_client.h	100.00% <ø> (ø)
...den/internal/golden_kitchen_sink_auth_decorator.cc	70.12% <0.00%> (-10.47%)	⬇️
...lden/internal/golden_kitchen_sink_auth_decorator.h	100.00% <ø> (ø)
...en/internal/golden_kitchen_sink_connection_impl.cc	76.13% <0.00%> (-14.41%)	⬇️
...den/internal/golden_kitchen_sink_connection_impl.h	77.77% <ø> (ø)
... and 19 more

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[coryan]

Do you want to add tests for the async cases? Or streaming?

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[dbolduc]

Do you want to add tests for the async cases? Or streaming?

I dropped the routing annotation on a streaming read and unary rpc where we generate async versions of the functions. IsContextMDValid revealed that if no routing matches were found, I wasn't sending any metadata at all. Yay for testing? Boo for self esteem.

[coryan]

I think we still need to figure out streaming writes and streaming read-write RPCs, but those are (a) very rare, and (b) already broken I think.