[AWS][API Gateway] Fix metric type

[constanca-m]

What does this PR do?

All metric fields are calculated through an aggregation (sum or avg). However, this is only for a given timestamp. On the next timestamp, new values will be used on the final aggregation and so on.

Because of this, the value for some field, 4XXError.sum for example, does not relate to the next 4XXError.sum field. The values are not cumulative over the different timestamps, and that is a requirement for a counter field.

All metric fields are gauge in this data stream.

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 #6293.

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• Start Time: 2023-07-14T07:16:35.101+0000

• Duration: 56 min 40 sec

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Package aws - 1.46.5 containing this change is available at https://epr.elastic.co/search?package=aws