add probe_ip_addr_hash to detect if the IP changes

[Wing924]

WHAT

add probe_ip_addr_hash metrics to specify the hash of IP address

WHY

We want to detect if the IP is changed by change(probe_ip_addr_hash[10h])

Use Case

Assume you use CDN and you want to monitoring your web server.

blackbox_exporter --> CDN edge server --> origin server

if the edge server failover, blackbox exporter returns error and the IP address will be changed.
With probe_ip_addr_hash, we can tell the error happen due to edge failover.

[brian-brazil]

Can we use a better hash?

[Wing924]

@brian-brazil this hash is copied from java's String.hashCode().
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/lang/String.java#l1452

Why I use this simple hash instead of SHA256:

• we don't need slow cryptographic hash function because IP address is not secret
• we can't set 256bit to value because value is float64
• unsigned 32bit integer looks good when you convert to float64

But I'm not have strong opinion on using this simple hash.
Do you have any recommendation?

[brian-brazil]

Use any hash that Go provides and truncate it to 32 bits.

[Wing924]

@brian-brazil I've changed the hash func to Go provided.

[brian-brazil]

This IP could have high cardinality, it's not safe to expose it.

[Wing924]

Understood. I've removed that.

[brian-brazil]

Thanks!

[basroovers]

Looking for some documentation on the metric on how this can be used lead me to this pull request.
I just wanted to share how an alert rule would work (1st post already showed it but this won't work since "change" is a non-existent function, "changes" is):

changes(probe_ip_addr_hash[10h])