Add numbackends to postgres dbstat metrics

As discussed in mperham/inspeqtor#26.  This makes the number of connections, or `numbackends` available when writing rules. For example, to check that your connection pool hasn't exploded unexpectedly:

```
if postgresql:numbackends > 50 for 4 cycles then alert
```

@mperham: No new tests were added with this, but the fixture was updated.