Previously it was both metadata about the class of warnable item as
well as the value.
Now it's only metadata and the value is per-Tracker.
Updates #11874
Updates #4136
Change-Id: Ia1ed1b6c95d34bc5aae36cffdb04279e6ba77015
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>