feat: add etcd otel content pack

[stefans-elastic]

Proposed commit message

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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.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

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Vale Linting Results

Summary: 2 warnings found

⚠️ Warnings (2)

File	Line	Rule	Message
packages/etcd_otel/docs/README.md	5	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'using' instead of 'via'.
packages/etcd_otel/docs/README.md	26	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'using' instead of 'via'.

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[elasticmachine]

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• 💔 Build #39619 failed dfbf7af