Add units to audience dashboard visualizations

What does this MR do and why?

Add units to audience dashboard visualizations in an attempt to make the visualizations easier to read & understand:

• Add "%" to returning user percentage
• Add "min" to average session duration

Note this feature is experimental and behind several feature flags.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard a project at Project -> Analyze -> Analytics dashboards.
3. View the Audience dashboard at at Project -> Analyze -> Analytics dashboards.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.