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
```
Treat numbackends as a guage, not a counter
Merge pull request #35 from sorentwo/postgres-numbackends

Add numbackends to postgres dbstat metrics