fix(pubsub): do not overwrite ConnectionOptions

[dbolduc]

When PublisherOptions is constructed, we default all options, including ones covered by ConnectionOptions, for example, GrpcCredentialOption.

google-cloud-cpp/google/cloud/pubsub/publisher_options.cc

Line 25 in c1a44cd

opts_ = pubsub_internal::DefaultPublisherOptions(std::move(opts));

google-cloud-cpp/google/cloud/pubsub/internal/defaults.cc

Line 113 in c1a44cd

return DefaultCommonOptions(std::move(opts));

This means that when we merge options here ....

google-cloud-cpp/google/cloud/pubsub/publisher_connection.cc

Lines 94 to 100 in c1a44cd

	std::shared_ptr<PublisherConnection> MakePublisherConnection(
	Topic topic, PublisherOptions options, ConnectionOptions connection_options,
	std::unique_ptr<pubsub::RetryPolicy const> retry_policy,
	std::unique_ptr<pubsub::BackoffPolicy const> backoff_policy) {
	auto opts = internal::MergeOptions(
	pubsub_internal::MakeOptions(std::move(options)),
	internal::MakeOptions(std::move(connection_options)));

.... we try to merge user-set options contained in ConnectionOptions but ignore them because those options all have default values from PublisherOptions.

(I am saying we, but this is solely @dbolduc's fault)

This is a disaster if users try to authenticate with custom credentials passed to ConnectionOptions. Also, the same is true for the Subscriber version of things.

To fix the problem, I could have just switched the order of the merge so that the PublisherOptions are merged into the ConnectionOptions. But I decided it would be better (and easier to test) that PublisherOptions only defaults options from the PublisherOptionList.

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This change is 

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[codecov]

Codecov Report

Merging #7406 (d1653e0) into main (39d87d4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7406   +/-   ##
=======================================
  Coverage   93.65%   93.65%           
=======================================
  Files        1351     1351           
  Lines      117334   117381   +47     
=======================================
+ Hits       109886   109935   +49     
+ Misses       7448     7446    -2     

Impacted Files	Coverage Δ
google/cloud/pubsub/internal/defaults.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/internal/defaults_test.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/publisher_options.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/publisher_options_test.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/subscriber_options.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/subscriber_options_test.cc	100.00% <100.00%> (ø)
...ud/spanner/integration_tests/client_stress_test.cc	85.52% <0.00%> (-0.66%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
google/cloud/grpc_error_delegate.cc	100.00% <0.00%> (+4.16%)	⬆️

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