identity/cache: only calculate String() for debug messages if debug=true
identity/cache: only calculate String() for debug messages if debug=true

It does not make sense to use logrus WithFields for Debug messages if
the values will never be used when debug is disabled.

Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

tgraf/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,cilium-team/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,cilium-team/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium