Provide sample code for tuning grpc parameters

[rbrtwng]

Cloud Bigtable allows user to override grpc parameters globally.

const bigtable = new Bigtable({
   'grpc.keepalive_time_ms': 30000,
   'grpc.keepalive_timeout_ms': 10000
});

And per offline conversation, for Cloud Spanner, you can't set these values at a global level. Instead these values can be passed with each function call. This is because for different queries number of retry values, timeout, etc make sense differently as per the operation.

Creating a FR to provide code sample in passing grpc parameters at function level.

[rahul2393]

@surbhigarg92 please help triage this ticket