Append customer prometheus scrape configs

[agarakan]

Description of the issue

When a customer provides prometheus scrape config relabel configs, we should not overwrite them. Additionally, the customer provided config must be processed last, after the default relabel config. This is to retain backwards compatibility given the new labels generated by the OTEL plugin for prometheus file-based service discovery (ECS Observer).

Description of changes

This change detects customer provided relabel configs and appends them to the dynamically configured relabel configs so that they can be processed after

ex:
Customer provides

		SourceLabels: model.LabelNames{"StartedBy"},
		TargetLabel:  "CustomStartedBy",
		Regex:        relabel.MustNewRegexp("(.*)"),

then the generated relabelConfig must look like this so that the Customer's action on StartedBy does not fail

...
Action:       relabel.Replace,
SourceLabels: model.LabelNames{"__meta_ecs_task_started_by"},
TargetLabel:  "StartedBy",
Regex:        relabel.MustNewRegexp("(.*)"),
...
Action:       relabel.Replace,
SourceLabels: model.LabelNames{"StartedBy"},
TargetLabel:  "CustomStartedBy",
Regex:        relabel.MustNewRegexp("(.*)"),

License

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Tests

Added unit tests to verify customer scrape configs & all unit tests are passing

make test
make fmt
make fmt-sh
make lint

Requirements

Before commiting your code, please do the following steps.

1. Run make fmt and make fmt-sh
2. Run make lint

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Integration Tests

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