UserFlow - show duration between timespan start and finish

**Feature request summary**

I would like the lighthouse User Flow HTML report to contain the duration between the timespan start and finish alongside the metrics. Currently there is only TBT, CLS and INP displayed in the timespan report meanwhile a duration would be useful especially that timespans are often used to calculate the time to get from one place on the website to a different one.
What do you think of such addition?

If you would like this feature then I think I would be able to contribute with the addition.

**What is the motivation or use case for changing this?**
More informative metrics on the LH User Flow Report.

**How is this beneficial to Lighthouse?**
A useful metric on the timespan report for LH User Flow Report.