[prometheus] Use ecs definition of the 'event.dataset' field

[tetianakravchenko]

What does this PR do?

1, There is no need to set event.dataset explicitly - this should be done on the elastic-agent side elastic/beats#20076.

due to the existence of predefined value, there can happen this error:

{"type":"illegal_argument_exception","reason":"[constant_keyword] field [event.dataset] only accepts values that are equal to the value defined in the mappings [prometheus.collector], but got [prometheus.topolvm]"}}, dropping event!

Since the type defined for this field in ecs is keyword, I decided to switch to ecs definition.

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.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes [prometheus] event.dataset is not correct if data_stream.dataset is set to not a default value #7654

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• Start Time: 2023-09-05T15:24:21.794+0000

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Package prometheus - 1.12.0 containing this change is available at https://epr.elastic.co/search?package=prometheus