Add http failure reason metric #1077

[domcyrus]

Adds probe_http_failures_total metric implementing feature for #1077 . This couter type metric provides the reason of http probe failures.

For example:

probe_http_failures_total{reason="dns_not_found"}
probe_http_failures_total{reason="request_creation_error"}
probe_http_failures_total{reason="request_get_timeout"}
...