[Istio] Update "Pilot xds expired" visualization

[constanca-m]

What does this PR do?

Update "Pilot xds expired" visualization to use last value instead of average.

"Pilot xds expired" visualization uses a counter metric, istio.istiod.metrics.pilot_xds_expired_nonce.counter. Currently, that metric is being used with an unsupported aggregation, average. Since we want to migrate this integration to TSDB we have to fix that. The solution is to use last_value and break the visualization by the label (see screenshot section).

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Related issues

Relates to #6567.

Screenshots

Currently the visualization looks like this:

With the change in this PR, it will look like this:

[gizas]

@constanca-m do we need to raise this to 8.8.0?

[constanca-m]

There is a warning message on #tsdb-observability channel that says this "It is an issue if an dashboard is exported using elastic-package in 8.8+ and the unsupported dashboard always results in a CI failure". That is why I migrated in 8.6.0 version (it was also the previous version being used) instead of 8.8.0 @gizas

[gizas]

Thanks have seen the relevant thread!

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• Start Time: 2023-07-19T07:35:36.198+0000

• Duration: 17 min 34 sec

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (3/3)	💚
Classes	100.0% (3/3)	💚
Methods	90.0% (27/30)	👎 -10.0
Lines	97.472% (347/356)	👎 -2.528
Conditionals	100.0% (0/0)	💚

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Package istio - 0.3.2 containing this change is available at https://epr.elastic.co/search?package=istio