plugin/metrics: add 'server' label (#1682)

* plugin/metrics: add 'server' label

This uses the new WithServer(ctx) to get the current server from the
context.

First in a larger refactor to make all plugins do this.

* compile

* compile

* lala test

* compile and test

* typos

* Dont duplicate the code

plugin/log/log.go

@@ -21,7 +21,7 @@ import (
 type Logger struct {
 	Next      plugin.Handler
 	Rules     []Rule
-	ErrorFunc func(dns.ResponseWriter, *dns.Msg, int) // failover error handler
+	ErrorFunc func(context.Context, dns.ResponseWriter, *dns.Msg, int) // failover error handler
 }
 
 // ServeDNS implements the plugin.Handler interface.
@@ -39,13 +39,13 @@ func (l Logger) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 			// There was an error up the chain, but no response has been written yet.
 			// The error must be handled here so the log entry will record the response size.
 			if l.ErrorFunc != nil {
-				l.ErrorFunc(rrw, r, rc)
+				l.ErrorFunc(ctx, rrw, r, rc)
 			} else {
 				answer := new(dns.Msg)
 				answer.SetRcode(r, rc)
 				state.SizeAndDo(answer)
 
-				vars.Report(state, vars.Dropped, rcode.ToString(rc), answer.Len(), time.Now())
+				vars.Report(ctx, state, vars.Dropped, rcode.ToString(rc), answer.Len(), time.Now())
 
 				w.WriteMsg(answer)
 			}