Export error and HTTP metrics

This change adds support of the following additional metrics:

* econ: haproxy_connection_errors
* eresp: haproxy_response_errors
* wretr: haproxy_retry_warnings
* wredis: haproxy_redispatch_warnings
* hrsp_{2xx,3xx,4xx,5xx}: haproxy_http_responses
Merge pull request #6 from prometheus/export-more-fields

Export error and HTTP metrics