When serializing msgpack, only consider codec tag

When serializing structs with msgpack, only consider type tags of
`codec`.

Hashicorp/go-msgpack (based on ugorji/go) defaults to interpretting
`codec` tag if it's available, but falls to using `json` if `codec`
isn't present.

This behavior is surprising in cases where we want to serialize json
differently from msgpack, e.g. serializing `ConsulExposeConfig`.

client/structs/structs.go

@@ -1,6 +1,6 @@
 package structs
 
-//go:generate codecgen -c github.com/hashicorp/go-msgpack/codec -d 102 -t codegen_generated -o structs.generated.go structs.go
+//go:generate codecgen -c github.com/hashicorp/go-msgpack/codec -st codec -d 102 -t codegen_generated -o structs.generated.go structs.go
 
 import (
 	"errors"

nomad/structs/generate.sh

@@ -4,6 +4,7 @@ set -e
 FILES="$(ls ./*.go | grep -v -e _test.go -e .generated.go | tr '\n' ' ')"
 codecgen \
     -c github.com/hashicorp/go-msgpack/codec \
+    -st codec \
     -d 100 \
     -t codegen_generated \
     -o structs.generated.go \

nomad/structs/structs.go

@@ -9561,6 +9561,9 @@ var MsgpackHandle = func() *codec.MsgpackHandle {
 	// nil interface{}.
 	h.MapType = reflect.TypeOf(map[string]interface{}(nil))
 
+	// only review struct codec tags
+	h.TypeInfos = codec.NewTypeInfos([]string{"codec"})
+
 	return h
 }()
 

plugins/base/plugin.go

@@ -65,6 +65,10 @@ var MsgpackHandle = func() *codec.MsgpackHandle {
 	h.BasicHandle.TimeNotBuiltin = true
 
 	h.MapType = reflect.TypeOf(map[string]interface{}(nil))
+
+	// only review struct codec tags - ignore `json` flags
+	h.TypeInfos = codec.NewTypeInfos([]string{"codec"})
+
 	return h
 }()