The TabManager.Discarding.DiscardCount and TabManager.Discarding.ReloadCount metrics record the number of discards/reloads since startup for every discard/reload event. Analyzing these histograms can be misleading, because they are affected by the lifetime of the browser process (metrics will inevitably be higher if Chrome runs for a long time), and they include both proactive and urgent discards.

We should have metrics that are easier to analyze, i.e.:
- Proactive and urgent discards are recorded to different histograms
- We record the number of discards per minute of Chrome usage, instead of number of discards since startup, to avoid being affected by the lifetime of the browser process.