Add zone serial numbers as metrics

[Seitanas]

This patch provides zone serial numbers as metrics:

bind_zone_serial{name="168.192.in-addr.arpa",rdataclass="IN",view="_default"} 1
bind_zone_serial{name="TEST_ZONE",rdataclass="IN",view="_default"} 123
bind_zone_serial{name="version.bind",rdataclass="CH",view="_bind"} 0

[Seitanas]

@SuperQ could you please review this one?

[SuperQ]

I'm not familiar with the rdataclass portion of the schema. Do you have any links to docs on what this is/for?

[Seitanas]

@SuperQ,

It is mentioned in https://www.ietf.org/rfc/rfc1035.txt

3.2.4. CLASS values

CLASS fields appear in resource records.  The following CLASS mnemonics
and values are defined:
IN              1 the Internet
CS              2 the CSNET class (Obsolete - used only for examples in
                some obsolete RFCs)
CH              3 the CHAOS class
HS              4 Hesiod [Dyer 87]

This is what you configure inside your zone definition:

zone "test.com" IN {   
file "test.zone";   
};

rdataclass will be equal to IN here.

I'm not sure this will be needed, but i didn't want to strip this label, as bind exports it.

[SuperQ]

Are we only concerned about zone serials? They would only have IN class. That would mean we could filter for only IN class entries. Then we could drop the label as rdataclass="IN" is implied.

[Seitanas]

I think you are right about serials being needed only for IN class.
Updated code to filter out other classes than IN

[SuperQ]

Minor nit, name is a bit generic, maybe zone or zone_name to make it more specific and less likely to conflict with other (discovery) labels?

[Seitanas]

Fixed. 👍

[SuperQ]

MInor nit, otherwise LGTM.

[SuperQ]

LGTM, Thanks!

[Seitanas]

@SuperQ,
what is release cycle of bind_exporter? Is it worth waiting for a binary release, or just use own compiled binaries?

[SuperQ]

We don't have a specific release cycle, but we can probably do one soon. We're rolling out the https library support, so maybe @roidelapluie would like to add that here.

I also have another bugfix PR open.