* Metrics: expand coredns_dns_responses_total with plugin label

This adds (somewhat hacky?) code to add a plugin label to the
coredns_dns_responses_total metric. It's completely obvlious to the
plugin as we just check who called the *recorder.WriteMsg method. We use
runtime.Caller( 1 2 3) to get multiple levels of callers, this should be
deep enough, but it depends on the dns.ResponseWriter wrapping that's
occuring.

README.md of metrics updates and test added in test/metrics_test.go to
check for the label being set.

I went through the plugin to see what metrics could be removed, but
actually didn't find any, the plugin push out metrics that make sense.

Due to the path fiddling to figure out the plugin name I doubt this
works (out-of-the-box) for external plugins, but I haven't tested that.
Signed-off-by: Miek Gieben <miek@miek.nl>

* better comment
Signed-off-by: Miek Gieben <miek@miek.nl>

* Metrics: expand coredns_dns_responses_total with plugin label

This adds (somewhat hacky?) code to add a plugin label to the
coredns_dns_responses_total metric. It's completely obvlious to the
plugin as we just check who called the *recorder.WriteMsg method. We use
runtime.Caller( 1 2 3) to get multiple levels of callers, this should be
deep enough, but it depends on the dns.ResponseWriter wrapping that's
occuring.

README.md of metrics updates and test added in test/metrics_test.go to
check for the label being set.

I went through the plugin to see what metrics could be removed, but
actually didn't find any, the plugin push out metrics that make sense.

Due to the path fiddling to figure out the plugin name I doubt this
works (out-of-the-box) for external plugins, but I haven't tested that.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Update core/dnsserver/server.go
Co-authored-by: dilyevsky <ilyevsky@gmail.com>

* Use [3]string
Signed-off-by: Miek Gieben <miek@miek.nl>

* imports
Signed-off-by: Miek Gieben <miek@miek.nl>

* remove dnstest changes
Signed-off-by: Miek Gieben <miek@miek.nl>

* revert
Signed-off-by: Miek Gieben <miek@miek.nl>

* Add some sleeps to make it less flaky
Signed-off-by: Miek Gieben <miek@miek.nl>

* Revert "Add some sleeps to make it less flaky"

This reverts commit b5c6655196e3ad570555f086832ceb1f48f6f2d5.

* Remove forward when not needed
Signed-off-by: Miek Gieben <miek@miek.nl>

* remove newline
Signed-off-by: Miek Gieben <miek@miek.nl>
Co-authored-by: dilyevsky <ilyevsky@gmail.com>