doc: add section to show how to customize cilium-agent metrics
doc: add section to show how to customize cilium-agent metrics

Signed-off-by: ArthurChiao <16a56a00a836623a07eccc04d896898d9695bb9c@hotmail.com>

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium