This bug was introduced in e6b84f215 (May 2020) but was only used in
tests when stringifying probeProto values on failure so it wasn't
noticed for a long time.
But then it was moved into non-test code in 8450a18aa (Jun 2024) and I
didn't notice during the code movement that it was wrong. It's still
only used in failure paths in logs, but having wrong/ambiguous
debugging information isn't the best.
Whoops.
Updates tailscale/corp#20654
Change-Id: I296c727ed1c292a04db7b46ecc05c07fc1abc774
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>