Enhance filter check in k8s event metricset

[ChrsMark]

What does this PR do?

Enhance filter check in k8s event metricset by checking eventTime field if lastTimestamp is not a valid one.

Why is it important?

Based on research made on #28923 lastTimestamp is possible to be null and hence we need to fallback to checking eventTime in order to filter older events. If both fields are null we cannot apply filtering and hence the skip_older: false should be used.

cc: @MichaelKatsoulis

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[mergify]

This pull request does not have a backport label. Could you fix it @ChrsMark? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[MichaelKatsoulis]

There is also time.IsZero() which checks if the time is 0001-01-01 00:00:00 +0000 UTC

[ChrsMark]

Good catch, that's a better check :).

[elasticmachine]

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• Start Time: 2021-12-16T14:20:07.092+0000

• Duration: 127 min 21 sec

• Commit: b96f541

Test stats 🧪

Test	Results
Failed	0
Passed	48046
Skipped	4148
Total	52194

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