[Spanner] GrpcTransport is created five times per request

[castaneai]

Hi,
I found that GrpcTransport is created five times per request.
According to my research, GrpcTransport::__construct is called from the following file:

https://github.com/googleapis/google-cloud-php/blob/master/Spanner/src/Connection/Grpc.php#L174

Google\Cloud\Spanner\Connection\Grpc::__construct creates the following four GrpcTransport instances for each call:

• SpannerGapicClient
• InstanceAdminGapicClient
• OperationsGapicClient from InstanceAdminGapicClient
• DatabaseAdminGapicClient
• OperationsGapicClient from DatabaseAdminGapicClient

Since web applications to be exposed to users do not require InstanceAdmin Client or DatabaseAdmin Client, generating these five transports may be a waste of work.
What do you think?