metrics: One relay metrics was not using metrics_format_label()

This resulted in the labels not being surrounded by double quotes and
thus Prometheus not liking it.

Signed-off-by: David Goulet <dgoulet@torproject.org>
9 jobs for relay-metrics-quick-fix2 in 26 minutes and 13 seconds (queued for 11 minutes and 55 seconds)
latest
Status Job ID Name Coverage
  Test
passed #20784
debian-disable-dirauth

00:05:51

passed #20785
debian-disable-relay

00:06:05

passed #20780
debian-distcheck

00:06:55

passed #20781
debian-docs

00:03:08

passed #20779
debian-hardened

00:07:11

passed #20782
debian-integration

00:14:55

passed #20778
debian-minimal

00:05:46

passed #20786
debian-nss

00:05:46

passed #20783
debian-tracing

00:07:58