Feature Request: expose metrics for failures / mismatch to spire entries

[Anhui-tqhuang]

if one ClusterStaticEntry exists, but we cannot find it in the spire-server for a long time (~10 mins), we need to expose some metrics lile

# HELP cluster_static_entry_failures Total number of cluster static entries that have no corresponding records in spire-server
# TYPE cluster_static_entry_failures gauge
cluster_static_entry_failures 10

or some other metrics which could indicate the failure