We were missing this metric, but it can be important for some workloads.
Varz memstats output allocation cost reduced from 30 allocs per
invocation to 1 alloc per invocation.
Updates tailscale/corp#28033
Signed-off-by: James Tucker <james@tailscale.com>
Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>