Standardize DD_TAGS format

[NouemanKHAL]

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• Fill out the template below. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• The pull request must only fix one issue, or add one feature, at the time.

What does this PR do?

This PR aims to add a standard interface to use the environment variableDD_TAGS in the buildpack regardless of the target language. As a result, this PR fixes all issues related to DD_TAGS format.

Description of the Change

• Add support for custom DD_TAGS separator via DD_TAGS_SEP
• Pass custom tags to the tracing libraries via a comma-separated DD_TAGS
• Pass custom tags to the agents via their yaml configuration file (tags, dogstatsd_tags)
• Fix a typo in DD_NODE_AGENT_TAGS env variable
• Fix tags parsing bugs

Alternate Designs

Possible Drawbacks

Verification Process

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• [ ] PR should not have do-not-merge/ label attached.
• [ ] If Applicable, issue must have kind/ and severity/ labels attached at least.

[NouemanKHAL]

Thank you @maycmlee !

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