New gauge reflects endpoints state via labels:
- open, when both peers are connected and ready to talk, and
- connecting. when at least one peer hasn't connected yet.
Corresponding client metrics are logged as
- udprelay_endpoints_connecting
- udprelay_endpoints_open
Updates tailscale/corp#30820
Change-Id: Idb1baa90a38c97847e14f9b2390093262ad0ea23
Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>
4b7585df77