[Prometheus] Revert prometheus metrics field definition to the format used before introducing metric_type

[tetianakravchenko]

What does this PR do?

Revert prometheus metrics field definition to the format used before introducing metric_type, as described in this comment. Due to this issue

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.

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• Start Time: 2023-08-09T12:40:57.550+0000

• Duration: 18 min 34 sec

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Passed	9
Skipped	0
Total	9

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (0/0)	💚
Files	100.0% (0/0)	💚
Classes	100.0% (0/0)	💚
Methods	72.727% (8/11)	👎 -26.81
Lines	100.0% (0/0)	💚 14.71
Conditionals	100.0% (0/0)	💚

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Package prometheus - 1.7.0 containing this change is available at https://epr.elastic.co/search?package=prometheus