The new metrics that have been added for the SessionPool are added multiple times if multiple different session pools are created by a Spanner instance. This could happen if the user creates database clients for different databases, or if multiple database clients using different SessionPoolOptions are requested. This again will lead to an InvalidArgumentException being thrown. See for example https://source.cloud.google.com/results/invocations/a8dc0694-336f-483b-bb8f-2ee502506a2b/targets/cloud-devrel%2Fjava%2Fjava-docs-samples%2Fjava8%2Fpresubmit/log