Roll out PackObjectsHookWithSidechannel

When #1126 (closed) is done, it should be possible to use a feature flag to enable PackObjectsHookWithSidechannel per project or a percentage of the time. This issue is a placeholder for planning the roll-out.

1. Update Gitaly latency SLI's so that they exclude PackObjectsHookWithSidechannel. Our latency SLI's exclude gRPC streaming RPC's by default, so also PackObjectsHook. But PackObjectsHookWithSidechannel is not labeled as a streaming RPC, it would get included in the latency SLI, and that would cause false alarms. gitlab-com/runbooks!3828 (merged)
2. Roll out in gstg production#5422 (closed)
3. Roll out in gprd production#5423 (closed)