Timeline tooltips of markers like LCP should include the values instead of timestamps

Currently an LCP tooltip looks like this:

Timestamp gives us very little information. It would be better to have the LCP value there instead. This is the same for other markers like FCP etc.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-1358)

The issue names no files or tests; locate the timeline marker tooltip entry point and inspect how LCP and FCP marker values and timestamps are supplied. Update the tooltip output so these markers show their values instead of only timestamps, then run the relevant timeline UI tests and verify the resulting tooltips.

Written by the indexing model from the issue text.