- `tailscale metrics print`: to show metric values in console
- `tailscale metrics write`: to write metrics to a file (with a tempfile
& rename dance, which is atomic on Unix).
Also, remove the `TS_DEBUG_USER_METRICS` envknob as we are getting
more confident in these metrics.
Updates tailscale/corp#22075
Signed-off-by: Anton Tolchanov <anton@tailscale.com>