docs: Escape templating syntax in metrics.md to avoid Jinja2 conflicts

[aman4433]

Escaped {{ $labels.* }} expressions in metrics.md using {{ "{{" }} syntax to prevent Jinja2 from rendering them during site build.
Fixes markdown build errors with mkdocs-macros.

fixes: #955

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PR needs rebase.

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[aman4433]

As already a similar pr is being merged, closing this PR