[AWS][Natgateway] Add metric type to fields

[constanca-m]

What does this PR do?

Add metric type to natgateway data stream fields, necessary to support TSDB in the future.

Details

Gauge and counter fields were defined based on:

1. The metrics that represent a specific value in time, without consideration for the previous one, are gauge.
2. If a metric keeps increment and decrementing, it is a gauge.
3. If a metric is cumulative, then it is a counter - but it can only increment.

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.

How to test this PR locally

Refer to #6101

Related issues

• Relates to [AWS][Nategateway] TSDB Enablement #6101

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• Start Time: 2023-05-22T08:58:03.786+0000

• Duration: 47 min 46 sec

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Total	192

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (15/15)	💚
Files	93.75% (15/16)	👎 -1.25
Classes	93.75% (15/16)	👎 -1.25
Methods	86.131% (236/274)	👎 -1.689
Lines	85.925% (7387/8597)	👎 -0.542
Conditionals	100.0% (0/0)	💚

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Package aws - 1.36.6 containing this change is available at https://epr.elastic.co/search?package=aws