prober: added ip address to metric label

[fluzzykitten]

What this PR does / Which issue(s) does the PR fix:

Adds IP address as label in returned metrics. Having the hash as a metric is lovely and all, but often you need the actual IP, which is now included as a label on metrics returned from probes.

Does this PR introduce a user-facing change?

Previously, metrics had the hash:

# HELP probe_ip_addr_hash Specifies the hash of IP address. It's useful to detect if the IP address changes.
# TYPE probe_ip_addr_hash gauge
probe_ip_addr_hash 3.521344457e+09
# HELP probe_ip_protocol Specifies whether probe ip protocol is IP4 or IP6
# TYPE probe_ip_protocol gauge
probe_ip_protocol 4
# HELP probe_success Displays whether or not the probe was a success
# TYPE probe_success gauge
probe_success 1

Now it also includes the actual IP:

# HELP probe_ip_addr IP address. It's useful.
# TYPE probe_ip_addr gauge
probe_ip_addr{ip="10.42.49.95"} 1

Checklist

• Tests updated
• Documentation added
• CHANGELOG added in release-notes section of PR Desc.