Add opt-in JSON logging with structured DNS query fields (#8553)

* plugin/pkg/log: add opt-in JSON logging backend

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

* plugin/log: emit typed query records in JSON mode

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

* coremain: expose process-wide JSON logging

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

---------

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

plugin/log/json.go

@@ -0,0 +1,44 @@
+package log
+
+import (
+	"log/slog"
+	"strconv"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	clog "github.com/coredns/coredns/plugin/pkg/log"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+func logJSON(msg string, state request.Request, rr *dnstest.Recorder) {
+	// A missing response is not NOERROR (for example an ACL drop). Deferred
+	// errors have already been synthesized by ServeDNS, just as in text mode.
+	rcode := slog.Any("rcode", nil)
+	if rr.Msg != nil {
+		rc := dns.RcodeToString[rr.Rcode]
+		if rc == "" {
+			rc = strconv.Itoa(rr.Rcode)
+		}
+		rcode = slog.String("rcode", rc)
+	}
+	port, _ := strconv.Atoi(state.Port())
+	clog.InfoAttrs(msg,
+		slog.String("plugin", "log"),
+		slog.String("client_ip", state.IP()),
+		slog.Int("client_port", port),
+		slog.String("qname", state.Name()),
+		slog.String("qtype", state.Type()),
+		slog.String("qclass", state.Class()),
+		slog.String("protocol", state.Proto()),
+		slog.Int("id", int(state.Req.Id)),
+		slog.Int("opcode", state.Req.Opcode),
+		slog.Int("request_size", state.Len()),
+		slog.Bool("dnssec_ok", state.Do()),
+		slog.Int("bufsize", state.Size()),
+		rcode,
+		slog.Int("response_size", rr.Len),
+		slog.Float64("duration_seconds", time.Since(rr.Start).Seconds()),
+	)
+}