feat: increase GRPC max metadata size to 4 MB

[pradn]

• Ack/modack operations on subscriptions with exactly-once delivery enabled may return up to 4 MB of metadata (about ack/modack success/failure)
• It's unclear what the current limit is (I searched far and wide), but it seems to be lower than 4 MB. So, we're currently seeing subscription StreamingPull stream closures as a result of this low limit. We aim to keep subscription the same for subscribers to exactly-once subscriptions, even if they don't use the new *with_response methods, so this fix ensures that.