Spanner is hanging when DCP is enabled

[sakthivelmanii]

More details about the bug GoogleCloudPlatform/DataflowTemplates#3204 (comment)

[damccorm]

Is there an external (or if not internal) place to track the issue in the client? That will allow us to keep up and remove builder.disableGrpcGcpExtension(); eventually

[sakthivelmanii]

@damccorm b/479954370