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.

[fluzzykitten]

Hi @brian-brazil @SuperQ , are you guys the codeowners/maintainers for the blackbox exporter?

This PR adds the resolved IP address as an additional label on probe metrics, which can be very helpful for debugging when DNS resolution varies or when monitoring anycast/CDN endpoints.

Would it be possible to get a review when you have a moment? Happy to make any adjustments needed.

Thanks!