chore: add last transition time metrics

[savme]

Summary

To support dashboarding of resource health over time we need to know when conditions last changed, not just their current value. This adds status_condition_last_transition_time gauge metrics for the four resources that previously only had status_condition coverage.

Changes

New generator blocks in config/resource-metrics-policies/networking-metrics.yaml:

Resource	Metric
Network	datum_cloud_network_status_condition_last_transition_time
Subnet	datum_cloud_subnet_status_condition_last_transition_time
SubnetClaim	datum_cloud_subnet_claim_status_condition_last_transition_time
HTTPProxy	datum_cloud_networking_http_proxy_status_condition_last_transition_time

Each block iterates status.conditions, emits double(timestamp(item.lastTransitionTime).getSeconds()), and carries name, namespace, condition, reason, status labels — consistent with the existing pattern on Domain, Gateway, and Backend.