The prefix is a signal to tsweb to treat this as a gauge metric when
generating the Prometheus version.
Signed-off-by: Mihai Parparita <mihai@tailscale.com>
These were supposed to be part of
3b541c833e but I guess I forgot to "git
add" them. Whoops.
Updates #3307
Change-Id: I8c768a61ec7102a01799e81dc502a22399b9e9f0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>