fix: update grpc.max_metadata_size to 4MiB for exactly-once, and shift ack/modack errors to 'debug' stream channel

[feywind]

As the title says, this sets a default value for grpc.max_metadata_size.

While fixing that, I discovered that we are incorrectly raising a client error for ack/modack failures. Rather than making these go silent for users (for debug purposes), I moved them to a 'debug' channel that can also be subscribed on Subscription instances.

Fixes #1504 🦕

[pradn]

Looks good. Please test:

1. subscribing for a long time from a exactly-once delivery-enabled subscription
2. subscribing from a regular subscription and turning on exactly-once for the subscription

In both cases, the subscribing should continue as normal.

[feywind]

Looks good. Please test:

1. subscribing for a long time from a exactly-once delivery-enabled subscription
2. subscribing from a regular subscription and turning on exactly-once for the subscription

In both cases, the subscribing should continue as normal.

These are tested and pass after the fix for the ack/modack errors.

[bcoe]

Left one nit that might be worth fixing.