update metrics catalog: add nginx.ssl.certificate.expiry.time metric

[vivki]

Proposed changes

Nginx-agent has been updated to expose nginx.ssl.certificate.expiry.time as a metric (disabled by default). NGINXaaS has enabled this metric for all customers. This PR updates the metrics catalog to include the new metric.

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
• My branch adheres to the Git conventions
• My content changes adhere to the F5 Technical Writing Style Guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

Footnotes

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩