add example for extending span attributes (#1936)

plugins/wasm-go/examples/custom-span-attribute/main.go

@@ -0,0 +1,70 @@
+// Copyright (c) 2022 Alibaba Group Holding Ltd.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/higress-group/proxy-wasm-go-sdk/proxywasm"
+	"github.com/higress-group/proxy-wasm-go-sdk/proxywasm/types"
+	"github.com/tidwall/gjson"
+
+	"github.com/alibaba/higress/plugins/wasm-go/pkg/wrapper"
+)
+
+func main() {
+	wrapper.SetCtx(
+		"custom-span-attribute",
+		wrapper.ParseConfigBy(parseConfig),
+		wrapper.ProcessRequestHeadersBy(onHttpRequestHeaders),
+	)
+}
+
+type CustomSpanAttributeConfig struct {
+	HeaderKey string
+}
+
+func setSpanAttribute(key string, value interface{}, log wrapper.Log) {
+	if value != "" {
+		traceSpanTag := wrapper.TraceSpanTagPrefix + key
+		if e := proxywasm.SetProperty([]string{traceSpanTag}, []byte(fmt.Sprint(value))); e != nil {
+			log.Warnf("failed to set %s in filter state: %v", traceSpanTag, e)
+		}
+	} else {
+		log.Debugf("failed to write span attribute [%s], because it's value is empty")
+	}
+}
+
+/*
+Example configuration:
+
+```yaml
+headerKey: foo
+```
+*/
+func parseConfig(configJson gjson.Result, config *CustomSpanAttributeConfig, log wrapper.Log) error {
+	config.HeaderKey = configJson.Get("headerKey").String()
+	if config.HeaderKey == "" {
+		return errors.New("no header key specified")
+	}
+	return nil
+}
+
+func onHttpRequestHeaders(ctx wrapper.HttpContext, config CustomSpanAttributeConfig, log wrapper.Log) types.Action {
+	headerValue, _ := proxywasm.GetHttpRequestHeader(config.HeaderKey)
+	setSpanAttribute(config.HeaderKey, headerValue, log)
+	return types.ActionContinue
+}