feat(spanner): remove dedefault dial option that disables service config

[mohanli-ml]

DirectPath based on Traffic Director can not work with grpc.WithDisableServiceConfig().
Background: googleapis/google-api-go-client#1260
Bug: b/202169093#comment8

[noahdietz]

This option is part of the generated codebase. This file should not be changed manually.

This was added to all cloud clients to prevent APIs from specifying retry policies that would retry in the grpc layer as well as in the gapic layer.

We can remove it from the generator if you believe this is no longer a concern.

[noahdietz]

Is this what you want? googleapis/gapic-generator-go#812

[mohanli-ml]

@noahdietz Thanks Noah! Yes I do want to remove grpc.WithDisableServiceConfig() for Spanner (and Bigtable, but Bigtable actually does not have this option so it is not a concern), as once it is set I do not have a way to remove it, so it must be removed.

However, from blame this option is set 2 years ago, and I am not sure if disabling retry in the grpc layer is still a concern. Do you know who has the context? If it is, then I guess we can either: 1). find another way to disable retry in grpc; 2). or add a new API WithEnableServiceConfig in grpc so that Spanner can set it for DirectPath traffic (@menghanl, Menghan, is it feasible to have WithEnableServiceConfig?)

[noahdietz]

I've merged the generator change. I will cut a release and update the generation pipeline today (hopefully). You can probably close this PR.

The option was added by default in anticipation of some changes that never came to fruition so I think its OK to remove it at this point.

[menghanl]

There's also an option to just disable retry: https://pkg.go.dev/google.golang.org/grpc#WithDisableRetry