Meter metric type

Add support for Meter metric type to the SDK.

Interface should be along the lines of:
```

void mark();
void mark(int n);

```


Compare to http://metrics.dropwizard.io/3.1.0/apidocs/com/codahale/metrics/Meter.html

Imported from Jira [BEAM-1613](https://issues.apache.org/jira/browse/BEAM-1613). Original Jira may contain additional context.
Reported by: aviemzur.