Migrate to slog

[majst01]

Also update to grpc-middleware v2

TODO:

• grpc metrics are still missing

[mwennrich]

(...)
# HELP go_memstats_sys_bytes Number of bytes obtained from system.
# TYPE go_memstats_sys_bytes gauge
go_memstats_sys_bytes 1.5832328e+07
# HELP go_threads Number of OS threads created.
# TYPE go_threads gauge
go_threads 9
# HELP grpc_server_handled_total Total number of RPCs completed on the server, regardless of success or failure.
# TYPE grpc_server_handled_total counter
grpc_server_handled_total{grpc_code="AlreadyExists",grpc_method="Create",grpc_service="v1.TenantService",grpc_type="unary"} 2
grpc_server_handled_total{grpc_code="FailedPrecondition",grpc_method="Update",grpc_service="v1.TenantService",grpc_type="unary"} 2
(...)