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/transport.go

@@ -0,0 +1,38 @@
+// 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 jaeger
+
+import (
+	"io"
+)
+
+// Transport abstracts the method of sending spans out of process.
+// Implementations are NOT required to be thread-safe; the RemoteReporter
+// is expected to only call methods on the Transport from the same go-routine.
+type Transport interface {
+	// Append converts the span to the wire representation and adds it
+	// to sender's internal buffer.  If the buffer exceeds its designated
+	// size, the transport should call Flush() and return the number of spans
+	// flushed, otherwise return 0. If error is returned, the returned number
+	// of spans is treated as failed span, and reported to metrics accordingly.
+	Append(span *Span) (int, error)
+
+	// Flush submits the internal buffer to the remote server. It returns the
+	// number of spans flushed. If error is returned, the returned number of
+	// spans is treated as failed span, and reported to metrics accordingly.
+	Flush() (int, error)
+
+	io.Closer
+}