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/thrift-gen/agent/constants.go

@@ -0,0 +1,23 @@
+// Autogenerated by Thrift Compiler (0.9.3)
+// DO NOT EDIT UNLESS YOU ARE SURE THAT YOU KNOW WHAT YOU ARE DOING
+
+package agent
+
+import (
+	"bytes"
+	"fmt"
+	"github.com/uber/jaeger-client-go/thrift"
+	"github.com/uber/jaeger-client-go/thrift-gen/jaeger"
+	"github.com/uber/jaeger-client-go/thrift-gen/zipkincore"
+)
+
+// (needed to ensure safety because of naive import list construction.)
+var _ = thrift.ZERO
+var _ = fmt.Printf
+var _ = bytes.Equal
+
+var _ = jaeger.GoUnusedProtection__
+var _ = zipkincore.GoUnusedProtection__
+
+func init() {
+}