swarm: integrate OpenTracing; propagate ctx to internal APIs (#17169)

* swarm: propagate ctx, enable opentracing

* swarm/tracing: log error when tracing is misconfigured

vendor/github.com/uber/jaeger-client-go/utils/http_json.go

@@ -0,0 +1,54 @@
+// Copyright (c) 2017 Uber Technologies, Inc.
+//
+// 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 utils
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+)
+
+// GetJSON makes an HTTP call to the specified URL and parses the returned JSON into `out`.
+func GetJSON(url string, out interface{}) error {
+	resp, err := http.Get(url)
+	if err != nil {
+		return err
+	}
+	return ReadJSON(resp, out)
+}
+
+// ReadJSON reads JSON from http.Response and parses it into `out`
+func ReadJSON(resp *http.Response, out interface{}) error {
+	defer resp.Body.Close()
+
+	if resp.StatusCode >= 400 {
+		body, err := ioutil.ReadAll(resp.Body)
+		if err != nil {
+			return err
+		}
+
+		return fmt.Errorf("StatusCode: %d, Body: %s", resp.StatusCode, body)
+	}
+
+	if out == nil {
+		io.Copy(ioutil.Discard, resp.Body)
+		return nil
+	}
+
+	decoder := json.NewDecoder(resp.Body)
+	return decoder.Decode(out)
+}