Refactor gen_metrics_html.py

What does this MR do?

This MR refactor gen_metrics_html.py to address the repetition problem (see the detailed comment here).

Specifically, it

1. extracts the repeated HTML code into a separate function.
2. fixes a bug where only the first record was processed, but it should have calculated the average across multiple records.

What are the relevant issue numbers?

Add performance benchmarks to scanner service a... (gitlab-org/gitlab#478056 - closed) • Hua Yan • 17.4 • At risk

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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• Ensure the report version matches the equivalent schema version
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer