Add deadline_type prometheus label

This adds the deadline_type prometheus label to counters from https://github.com/grpc-ecosystem/go-grpc-prometheus

This was required for gitlab!21492 (merged), in which we will limit the deadline in function of the worker timeout: That way a gRPC call won't exceed the worker timeout. When the deadline is limited like this, there will be a metadata header called deadline_type with the value limited, otherwise the value is regular.

When the deadline is limited Gitaly could be left with an unreasonably short time to complete the request. Which would result in a DeadlineExceeded grpc-error. To be able to distinguish those in the metrics used for alerting, we'll use this new label.