Process metrics added

[gfukushima]

Pull request checklist

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Before creating the PR

Make sure that:

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Describe the change

Added an additional "Process metrics" table to the available metrics section with the new metric and its definition.

Issue fixed

See #480

Impacted parts

For content changes:

• Documentation content
• Documentation page organization

For tool changes:

• CircleCI workflow
• Build and QA tools (for example, lint or vale)
• MkDocs templates
• MkDocs configuration
• Python dependencies
• Node dependencies and JavaScript
• Read the Docs configuration
• GitHub integration

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• You've fixed any issues raised by the tests.
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Preview

[CLAassistant]

All committers have signed the CLA.

[siladu]

LGTM

[rolandtyler]

lgtm