Add support for TRACE, CONNECT and unrecognised verbs

We're currently seeing an exception from the metrics.ring.instrument namespace whenever we receive a request using the HTTP verbs TRACE or CONNECT:

```
java.lang.NullPointerException: null
	at metrics.ring.instrument$instrument$fn__4119.invoke(instrument.clj:43)
```

This change adds these two verbs, but also ensures that any more unrecognised verbs can be handled, by logging them all under an 'OTHER' metric.