fix: Correct the field mask when adding labels to an instance that had none previously

[dazuma]

Protobuf 4.x had a breaking change that affected how proto messages are converted to hashes. This affected the field mask computation when setting instance attributes—in particular, when using protobuf 4.x, it was no longer possible to set labels on an instance that had none previously because the field mask was omitting the labels field. This fix updates the field mask computation to be immune to the change.

[aandreassa]

I think this needs to be regenerated, similar to googleapis/google-cloud-ruby#26510

/workspace/google-cloud-spanner/benchmark/benchwrapper/spanner_pb.rb:20:in `': undefined method `build' for # (NoMethodError) Google::Protobuf::DescriptorPool.generated_pool.build do` 

[aandreassa]

LGTM. Just some other error on the build now. Something like:
/usr/grte/v5/lib64/libc.so.6: version GLIBC_ABI_DT_RELR' not found`

[dazuma]

LGTM. Just some other error on the build now. Something like: /usr/grte/v5/lib64/libc.so.6: version GLIBC_ABI_DT_RELR' not found`

Yeah that's been there for a while on that job. We may need help from the spanner team, as I'm not sure what that job is.