ci: reduce spanner CreateInstance rpcs in daily build

[dbolduc]

We have been seeing flakes in our integration-daily build.

e.g. https://console.cloud.google.com/cloud-build/builds;region=us-east1/2f83568c-9612-44d5-aafe-5c03e9f7a319?project=936212892354

google/cloud/spanner/admin/integration_tests/instance_admin_integration_test.cc:187: Failure
Value of: instance
Expected: code is equal to OK and message is anything
  Actual: 176-byte object <00-46 02-C8 BC-7F 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 ... 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00>, whose status is RESOURCE_EXHAUSTED: Error in non-idempotent operation: Quota exceeded for quota metric 'Instance create requests' and limit 'Instance create requests per minute' of service 'spanner.googleapis.com' for consumer 'project_number:936212892354'. error_info={reason=RATE_LIMIT_EXCEEDED, domain=googleapis.com, metadata={gcloud-cpp.retry.reason=non-idempotent, gcloud-cpp.retry.original-message=Quota exceeded for quota metric 'Instance create requests' and limit 'Instance create requests per minute' of service 'spanner.googleapis.com' for consumer 'project_number:936212892354'., quota_location=global, gcloud-cpp.retry.function=CreateInstance, service=spanner.googleapis.com, quota_limit=AdminInstanceCreateMethodQuotaPerMinutePerProject, quota_metric=spanner.googleapis.com/admin_instance_create_method_quota, quota_limit_value=5, consumer=projects/936212892354}}, with a code that isn't equal to OK, but a message that is anything

So reduce the CreateInstance calls that run against prod. In testing GAPIC LRO's we will prefer to use CreateDatabase instead of CreateInstance.

The gRPC transcoding tests are left as is to keep their coverage the same.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.59%. Comparing base (e353a33) to head (f9e25e4).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14468   +/-   ##
=======================================
  Coverage   93.59%   93.59%           
=======================================
  Files        2315     2315           
  Lines      207140   207140           
=======================================
  Hits       193869   193869           
  Misses      13271    13271           

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I changed the code too much since this review.

[scotthart]

Reviewed 3 of 3 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dbolduc)