fix: add keepalive to gRPC channel (#49)

Co-authored-by: larkee <larkee@users.noreply.github.com>

Author: larkee

google/cloud/spanner_v1/gapic/transports/spanner_grpc_transport.py

@@ -23,6 +23,7 @@
 from google.cloud.spanner_v1.proto import spanner_pb2_grpc
 
 
+_GRPC_KEEPALIVE_MS = 2 * 60 * 1000
 _SPANNER_GRPC_CONFIG = "spanner.grpc.config"
 
 
@@ -73,6 +74,7 @@ def __init__(
  options={
  "grpc.max_send_message_length": -1,
  "grpc.max_receive_message_length": -1,
+ "grpc.keepalive_time_ms": _GRPC_KEEPALIVE_MS,
  }.items(),
  )