Add agent health during merge

[jefchien]

Description of the issue

Following #2029, we want to retain the UserAgent attached to AWS requests.

Description of changes

• Adds a new mergeAgentHealth that gets run on the result of the OTEL YAML merge.
• Moves mergeConfigs into a separate merge.go.

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

Built and tested against a simple configuration

exporters:
    awscloudwatch:
        force_flush_interval: 1m0s
        max_datums_per_call: 1000
        max_values_per_datum: 150
        namespace: TestMerge
        region: us-east-1
        resource_to_telemetry_conversion:
            enabled: true
receivers:
  otlp:
    protocols:
      grpc:
        endpoint: localhost:4317
      http:
        endpoint: localhost:4318
service:
  pipelines:
    metrics:
      receivers: [otlp]
      exporters: [awscloudwatch]

With AWS SDK debug logging on

2026-02-27T18:52:49Z I! DEBUG: Request monitoring/PutMetricData Details:
---[ REQUEST POST-SIGN ]-----------------------------
POST / HTTP/1.1
Host: monitoring.us-east-1.amazonaws.com
User-Agent: CWAgent/1.300064.1-15-gefad4e835-dirty-untracked (go1.25.7; linux; amd64) ID/<snip> inputs:(otlp) outputs:(awscloudwatch) feature:(otel_merge_yaml) aws-sdk-go/1.48.6 (go1.25.7; linux; amd64)
X-Amz-Agent-Stats: "cpu":0.6,"mem":105365504,"fd":12,"th":8,"ric":0

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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