Marker usage scenario: compute average time used for a certain marker

Not sure if this is already on file, or if we have an organized way of collecting marker usage scenario somewhere. I'm filing this so that we have it written down somewhere.

aosmond had the following use case for the marker table recently:

He wanted to measure the average time to allocate a buffer for a frame of an animated image. In order to do that, he added a marker for every allocation. The thread he was looking at had a period during which it was constantly busy allocating these frames, so he could get the total duration spent in the markers by selecting the right range in the thread, but he couldn't see the number of markers in that range in order to calculate the average duration.

One quick way of helping with this use case would be to display the number of markers in the marker table somewhere.
Another thing that would work would be to allow multiselect in the marker table, and then allow copying out an array of marker data JSON, so that the count can be computed by looking at that array.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-35)