Add external_metrics_updated in DCA telemetry to check when we last received data from backend for it

[vboulineau]

What does this PR do?

Add mapping for new metric in https://github.com/DataDog/datadog-agent/pull/22103

Motivation

Additional Notes

Review checklist (to be filled by reviewers)

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.60%. Comparing base (a368484) to head (2305fad). Report is 480 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
confluent_platform	?
datadog_cluster_agent	90.19% <ø> (ø)
hive	?
hivemq	?
hudi	?
ignite	?
jboss_wildfly	?
presto	?
solr	?
tomcat	?
weblogic	?

Flags with carried forward coverage won't be shown. Click here to find out more.

[github-actions[bot]]

Test Results

4 files  4 suites   7s :stopwatch: 2 tests 2 :white_check_mark: 0 :zzz: 0 :x: 6 runs  4 :white_check_mark: 2 :zzz: 0 :x:

Results for commit 2305fad7.