(This is used to request new product features, please visit https://github.com/istio/istio/discussions for questions on using Istio)

Describe the feature request

Currently istio metrics citadel_server_cert_chain_expiry_timestamp reports The unix timestamp, in seconds, when Citadel cert chain will expire. A negative time indicates the cert is expired..
It is not possible in datadog to create alert for cert expiration is due in x days, as to get time left we need to subtract current time from metrics value. ( not supported functionality in datadog)

https://istio.io/latest/docs/reference/commands/pilot-discovery/#metrics

same case is for citadel_server_root_cert_expiry_timestamp metrics

Describe alternatives you've considered

we can add additional metrics citadel_server_cert_chain_expiry_time_left that report time in seconds left for expiration of certificate along with current metrics .

same case is for citadel_server_root_cert_expiry_time_left metrics.

Affected product area (please put an X in all that apply)

[ ] Ambient
[ ] Docs
[ ] Dual Stack
[ ] Installation
[ ] Networking
[ ] Performance and Scalability
[*] Extensions and Telemetry
[ ] Security
[ ] Test and Release
[ ] User Experience
[ ] Developer Infrastructure

Affected features (please put an X in all that apply)

[ ] Multi Cluster
[ ] Virtual Machine
[ ] Multi Control Plane

Additional context