Change format for Performance based metrics in scheduled reports

[chippison]

Description

UX-354

We have identified 7 Performance metrics that we will reformat to be in 'human-readable' form
These are:

• avg_time_network
• avg_time_server
• avg_time_transfer
• avg_time_dom_processing
• avg_time_dom_completion
• avg_time_on_load
• avg_page_load_time

These metrics will now be formatted to be 'x mins y s' or 'z hours x mins y s'

Checklist

• [NA] I have understood, reviewed, and tested all AI outputs before use
• [NA] All AI instructions respect security, IP, and privacy rules

Review

• Functional review done
• Potential edge cases thought about (behaviour of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• New documentation added or existing documentation updated