Use Metadata supplied by client library

[apstndb]

Merge after google-cloud-go/spanner has been released.
Use spanner/v1.12.0 and show Metadata at any time.

$ go run ./ ${DATABASE_ID} --query-mode=PLAN --sql='SELECT @str.*' --param='str:STRUCT<FirstName STRING, LastName STRING>' --format=yaml --filter='.metadata'
rowType:
  fields:
  - name: FirstName
    type:
      code: STRING
  - name: LastName
    type:
      code: STRING
transaction:
  readTimestamp: 2020-10-23T03:07:29.088435Z

refs googleapis/google-cloud-go#1805