Deriving values for an error.kind label

[objectiser]

As discussed in https://github.com/opentracing-contrib/java-metrics/issues/1#issuecomment-308471323, it would be good to have an error.kind label that provides more detail about the nature of an error (if one occurred).

If the span relates to HTTP, then it could be the status code, or a summary value (e.g. 4xx, 5xx).

Need to consider what other standard values may apply, and whether they equally have a detailed and summary form.