The inflight request tracker only starts recording a new bucket after
the first non-error request. Unfortunately, it's written in such a way
that ONLY successful requests are ever marked as being finished. Once a
bucket has had at least one successful request and begun to be tracked,
all subsequent error cases are never marked finished and always appear
as in-flight.
This change ensures that if a request is recorded has having been
started, we also mark it as finished at the end.
Updates tailscale/corp#19767
Signed-off-by: Will Norris <will@tailscale.com>