Plugin dnstap: add support for "extra" field in payload (#6226)

* dnstap: add 'extra' field

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* dnstap: add setup_test for 'extra' field

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* udnstap: update document and test

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* dnstap: update setup_test for more coverage

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* dnstap: add TapMessageWithMetadata function to Dnstap

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* dnstap: adapt dnstap and forward plugins to use TapMessageWithMetadata

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* change TapMessageWithMetadata function

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* tab inconsistency fix

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* fix replacer to support empty state

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* add replacer test for empty status parameter

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* dnstap: update unit test for 'extra' field

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* clean up code

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* gofmt fix & static analysis fix

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

* dnstap: refactor

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

---------

Signed-off-by: chenyuheng <chenyuheng99@qq.com>

plugin/dnstap/README.md

@@ -18,6 +18,7 @@ Every message is sent to the socket as soon as it comes in, the *dnstap* plugin
 dnstap SOCKET [full] {
   [identity IDENTITY]
   [version VERSION]
+  [extra EXTRA]
   [skipverify]
 }
 ~~~
@@ -26,6 +27,7 @@ dnstap SOCKET [full] {
 * `full` to include the wire-format DNS message.
 * **IDENTITY** to override the identity of the server. Defaults to the hostname.
 * **VERSION** to override the version field. Defaults to the CoreDNS version.
+* **EXTRA** to define "extra" field in dnstap payload, [metadata](../metadata/) replacement available here.
 * `skipverify` to skip tls verification during connection. Default to be secure
 
 ## Examples
@@ -63,6 +65,16 @@ dnstap /tmp/dnstap.sock {
 }
 ~~~
 
+Log to a socket, customize the "extra" field in dnstap payload. You may use metadata provided by other plugins in the extra field.
+
+~~~ txt
+forward . 8.8.8.8
+metadata
+dnstap /tmp/dnstap.sock {
+  extra "upstream: {/forward/upstream}"
+}
+~~~
+
 Log to a remote TLS endpoint.
 
 ~~~ txt
@@ -124,7 +136,9 @@ And then in your plugin:
 
 ~~~ go
 import (
-  github.com/coredns/coredns/plugin/dnstap/msg
+  "github.com/coredns/coredns/plugin/dnstap/msg"
+  "github.com/coredns/coredns/request"
+
   tap "github.com/dnstap/golang-dnstap"
 )
 
@@ -138,7 +152,12 @@ func (x ExamplePlugin) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dn
             q.QueryMessage = buf
         }
         msg.SetType(q, tap.Message_CLIENT_QUERY)
+        
+        // if no metadata interpretation is needed, just send the message
         tapPlugin.TapMessage(q)
+
+        // OR: to interpret the metadata in "extra" field, give more context info
+        tapPlugin.TapMessageWithMetadata(ctx, q, request.Request{W: w, Req: query})
     }
     // ...
 }