Merged threads should show originating thread in tooltip

https://share.firefox.dev/3t1SsS2

My use case was I was profiling a new feature I'm working on, that calls out into OS-libraries on a background thread. I didn't really know where and what this work was doing, so I recorded all threads, tediously cmd clicked all the threads in the process for a merged view, then I started dropping samples until I found the code I was looking for. After this, I couldn't find where my samples were coming from, and I wanted to know the specific thread. It would be nice if this information were in the tooltip or sidebar.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-529)