Observability: Praefect Gitaly client needs to set correlation ID's

Praefect contains a Gitaly client that it uses to make ReplicateRepository calls. That client should set correlation ID's in the outbound gRPC metadata and I believe that currently it does not.

This is particularly important because ReplicateRepository makes further RPC calls itself, and without correlation ID's we can't trace those.

It's probably a matter of adding the right labkit middleware?