pgbouncer in GPRD has an invalid user in SHOW POOLS

While testing the pgbouncer exporter against GPRD, I noticed it was failing hard here:

```golang
column name is user, data is ˇˇˇˇC
panic: label value "\xff\xff\xff\xffC" is not valid UTF-8

goroutine 19 [running]:
github.com/prometheus/client_golang/prometheus.MustNewConstMetric(0xc420147500, 0x2, 0x0, 0xc420180340, 0x2, 0x2, 0x0, 0x0)
        /home/stanhu/go/src/github.com/prometheus/client_golang/prometheus/value.go:98 +0xb0
main.queryNamespaceMapping(0xc42012c600, 0xc420110c80, 0x8f7bd9, 0x5, 0xc42011eed0, 0x0, 0x0, 0x0, 0x0, 0x0)
        /home/stanhu/go/src/github.com/larseen/pgbouncer_exporter/collector.go:146 +0xa51
main.queryNamespaceMappings(0xc42012c600, 0xc420110c80, 0xc42011eba0, 0xc420116e60)
        /home/stanhu/go/src/github.com/larseen/pgbouncer_exporter/collector.go:229 +0x213
main.(*Exporter).scrape(0xc42015c900, 0xc42012c600)
        /home/stanhu/go/src/github.com/larseen/pgbouncer_exporter/collector.go:295 +0x18c
main.(*Exporter).Collect(0xc42015c900, 0xc42012c600)
        /home/stanhu/go/src/github.com/larseen/pgbouncer_exporter/collector.go:276 +0x3c
main.(*Exporter).Describe(0xc42015c900, 0xc42012c5a0)
        /home/stanhu/go/src/github.com/larseen/pgbouncer_exporter/collector.go:269 +0xb0
github.com/prometheus/client_golang/prometheus.(*Registry).Register.func1(0xb09b80, 0xc42015c900, 0xc42012c5a0)
        /home/stanhu/go/src/github.com/prometheus/client_golang/prometheus/registry.go:250 +0x3b
created by github.com/prometheus/client_golang/prometheus.(*Registry).Register
```

If you look at `SHOW POOLS`, you see the `C` under `gitlabhq_production_sidekiq`. I haven't checked production yet, but I wonder where this garbage comes from:

```sql
root@pgbouncer-01-db-gprd.c.gitlab-production.internal:/tmp# sudo -u gitlab-psql /opt/gitlab/embedded/bin/psql -h /tmp -p 6432 -U pgbouncer -d pgbouncer
psql (9.6.5, server 1.7.2/bouncer)
Type "help" for help.

pgbouncer=# show pools;
          database           |      user      | cl_active | cl_waiting | sv_active | sv_idle | sv_used | sv_tested | sv_login | maxwait |  pool_mode  
-----------------------------+----------------+-----------+------------+-----------+---------+---------+-----------+----------+---------+-------------
 gitlabhq_production         | chatops        |         0 |          0 |         0 |       0 |       0 |         0 |        0 |       0 | transaction
 gitlabhq_production         | gitlab         |       274 |          0 |         0 |       4 |       1 |         0 |        0 |       0 | transaction
 gitlabhq_production         | gitlab_geo_fdw |        21 |          0 |         7 |       6 |       4 |         0 |        0 |       0 | transaction
 gitlabhq_production         | pgbouncer      |         0 |          0 |         0 |       1 |       0 |         0 |        0 |       0 | transaction
 gitlabhq_production_sidekiq | gitlab         |         0 |          0 |         0 |       0 |       0 |         0 |        0 |       0 | transaction
 gitlabhq_production_sidekiq | pgbouncer      |         0 |          0 |         0 |       0 |       0 |         0 |        0 |       0 | transaction
 gitlabhq_production_sidekiq | C              |         0 |          0 |         0 |       0 |       0 |         0 |        0 |       0 | transaction
 pgbouncer                   | pgbouncer      |         2 |          0 |         0 |       0 |       0 |         0 |        0 |       0 | statement
(8 rows)
```

Story points: 1.0