fix(common): avoid globals for easier DLLs

[coryan]

CMake can automatically create Windows DLLs if we don't use data symbols in the library. This may unblock some folks trying to support DLLs.

Fixes #10205

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: c3aa4c9f50ccd3d96fc9773d82f26f4f43116933

• Public Access Build Logs
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ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Base: 93.98% // Head: 93.98% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (c3aa4c9) compared to base (f617e71).
Patch coverage: 87.50% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10212   +/-   ##
=======================================
  Coverage   93.98%   93.98%           
=======================================
  Files        1531     1532    +1     
  Lines      141082   141084    +2     
=======================================
+ Hits       132589   132597    +8     
+ Misses       8493     8487    -6     

Impacted Files	Coverage Δ
google/cloud/grpc_error_delegate.cc	93.42% <ø> (ø)
google/cloud/spanner/internal/status_utils.cc	0.00% <0.00%> (ø)
google/cloud/grpc_error_delegate_test.cc	100.00% <100.00%> (ø)
google/cloud/internal/log_wrapper_helpers.cc	83.16% <100.00%> (ø)
google/cloud/internal/status_payload_keys.cc	100.00% <100.00%> (ø)
google/cloud/internal/status_payload_keys_test.cc	100.00% <100.00%> (ø)
google/cloud/spanner/internal/status_utils_test.cc	100.00% <100.00%> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	98.65% <0.00%> (+0.83%)	⬆️
...integration_tests/schema_admin_integration_test.cc	100.00% <0.00%> (+1.11%)	⬆️

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