fix(generator): handle explicit routing params for nested fields

[dbolduc]

Part of the work for #9121

Handle fields that look like field.sub_field. I re-read the explicit routing parameters spec and they allow for these nested fields.

We copy the implementation from the HttpRule:

google-cloud-cpp/generator/internal/descriptor_utils.cc

Lines 255 to 264 in d67dce6

	std::vector<std::string> chunks;
	auto const* input_type = method.input_type();
	for (auto const& sv : absl::StrSplit(param, '.')) {
	auto const chunk = std::string(sv);
	auto const* chunk_descriptor = input_type->FindFieldByName(chunk);
	chunks.push_back(FieldName(chunk_descriptor));
	input_type = chunk_descriptor->message_type();
	}
	method_vars["method_request_param_value"] =
	absl::StrJoin(chunks, "().") + "()";

In order to test this, IsContextMDValid needs to learn how to generically access this field name. (It did not need to before because the HttpRule only checks that a routing key exists and matches some known pattern. It does not set its expectations by looking at the request's contents like the RoutingRule does.)

I added a new test for the nested case. And updated all explicit routing tests to both use IsContextMDValid and to verify the expected headers manually. This sorta-kinda tests IsContextMDValid.

---

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 77fa685de25b45a41206ab3873ab762767e1947a

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Merging #9408 (c739b3b) into main (d67dce6) will increase coverage by 0.00%.
The diff coverage is 98.70%.

@@           Coverage Diff           @@
##             main    #9408   +/-   ##
=======================================
  Coverage   94.60%   94.61%           
=======================================
  Files        1485     1485           
  Lines      136008   136054   +46     
=======================================
+ Hits       128669   128721   +52     
+ Misses       7339     7333    -6     

Impacted Files	Coverage Δ
...egration_tests/golden/golden_kitchen_sink_client.h	100.00% <ø> (ø)
google/cloud/testing_util/validate_metadata.cc	92.56% <90.90%> (+1.07%)	⬆️
...internal/golden_kitchen_sink_metadata_decorator.cc	88.18% <100.00%> (+1.14%)	⬆️
...sts/golden_kitchen_sink_metadata_decorator_test.cc	100.00% <100.00%> (ø)
generator/internal/descriptor_utils.cc	88.22% <100.00%> (+0.13%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	97.05% <0.00%> (-2.95%)	⬇️
...integration_tests/schema_admin_integration_test.cc	98.88% <0.00%> (-1.12%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (-0.57%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.50% <0.00%> (+0.74%)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d67dce6...c739b3b. Read the comment docs.

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: cc5f8d9c7bde315ef0f6126edf879c42fd8faa47

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: c739b3bae565a4ac74fd6e0c4cebc3cae891b1fa

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.