feat: add config.tags and +tags support for metrics

[dtaniwaki]

Resolves #12607

Thanks for maintaining dbt-core! I'd appreciate your review on this PR.

Problem

Metrics do not support config.tags in YAML or +tags in dbt_project.yml. There is no way to apply tags to metrics through the standard config system, making it inconsistent with other resource types.

Additionally, v1 metrics had a top-level tags: field that was silently applied to the node outside of the config system, which is inconsistent with how other resource types handle tags.

Solution

Add tags to MetricConfig using MergeBehavior.Append, so tags from dbt_project.yml (+tags) and from config.tags in YAML are merged, deduplicated, and sorted.

YAML config block:

metrics:
  - name: my_metric
    ...
    config:
      tags:
        - finance

dbt_project.yml:

metrics:
  my_project:
    +tags:
      - finance

Top-level tags: on v1 metrics (outside of a config: block) now emit a ValidationWarning directing users to use config.tags instead, since metrics implement tags via the config system only.

A normalize_tags() helper is introduced in parser/common.py to deduplicate and sort tags consistently, and is reused across ExposureParser, MetricParser, and SavedQueryParser.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

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