dnsdist: Don't use square brackets for IPv6 in Carbon metrics

[rgacogne]

Short description

Use 2001:db8::1:53_tcp instead of [2001:db8::1]:53_tcp, as square brackets have special meaning for some Carbon storage engines and backends. I went with the :port instead of _port as suggested in the initial report to be consistent with the current IPv4 encoding. Since the port is always present it shouldn't be ambiguous, but I'm of course open to switch to another encoding.

Fixes #5538.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[pieterlexis]

lgtm!