allow metrics to be numbers instead of strings

currently it's not possible to generate a metric whose key is a number instead of a string.

This would be useful, for example, in the logstash-output-elasticsearch to record counts of events bucketed by response codes. Right now, these numbers [have to be converted into strings](https://github.com/logstash-plugins/logstash-output-elasticsearch/pull/585/files#diff-8eecbfb9f47550237e76ea104f7156b5R146)