feat: add istio citadel_server_cert_chain_expiry_timestamp metric

[jwillker]

What does this PR do?

This PR adds a new citadel metric, citadel_server_cert_chain_expiry_timestamp metric, to Istio integration.

Fix: #17267

Motivation

We need this metric in Datadog integration so we can create monitors when the certs expire and so on.

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

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Changelog entries must be created for modifications to shipped code
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.

[github-actions]

Test Results

 3 files   3 suites   5s ⏱️
18 tests 16 ✅  1 💤 1 ❌
38 runs  20 ✅ 17 💤 1 ❌

For more details on these failures, see this check.

Results for commit d330b0f.

♻️ This comment has been updated with latest results.

[FlorentClarret]

Hey @jwillker!

Thank you very much for this PR. I pushed a couple of tiny changes to our test suite and metadata, I hope you don't mind.

Your modifications looks good to me, I'll merge this as soon as the CI is green. This will be shipped by default with Agent 7.54.0. If needed, we can release the integration with this change if you're willing to manually install it on your Agents.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.48%. Comparing base (3bd07bb) to head (d330b0f).
Report is 12 commits behind head on master.

❗ Current head d330b0f differs from pull request most recent head 12ae791. Consider uploading reports for the commit 12ae791 to get more accurate results

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
confluent_platform	?
hive	?
hivemq	?
hudi	?
ignite	?
istio	78.14% <ø> (+0.51%)	⬆️
jboss_wildfly	?
kafka	?
presto	?
solr	?
weblogic	?

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