add span per connection attempt

[Tantalor93]

Signed-off-by: Ondrej Benkovsky ondrej.benkovsky@wandera.com

Hello, I would like to improve the observability of fanout plugin by adding opentracing spans per upstream connection attempt similarly as CoreDNS forward plugin does. You can then better see the performance of each DNS upstream nameserver

Example how it looks like: Let's have a CoreDNS instance running with Corefile

.:53 {
    fanout . 8.8.8.8 9.9.9.9
    trace zipkin 127.0.0.1:9411 {
              service dns-service
    }
}

and locally running Zipkin in docker

docker run -d -p 9411:9411 openzipkin/zipkin

when you do the DNS request through this CoreDNS instance

dig @127.0.0.1 example.com

it will create a trace such as this (screen from Zipkin UI)