Tracing: Support configuring Jaeger client from environment (#21103)

* Tracing: Support configuring Jaeger client from environment
* Tracing: Replace deprecated Jaeger Client call

vendor/github.com/uber/jaeger-client-go/span_allocator.go

@@ -0,0 +1,56 @@
+// Copyright (c) 2019 The Jaeger Authors.
+//
+// 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 "sync"
+
+// SpanAllocator abstraction of managign span allocations
+type SpanAllocator interface {
+	Get() *Span
+	Put(*Span)
+}
+
+type syncPollSpanAllocator struct {
+	spanPool sync.Pool
+}
+
+func newSyncPollSpanAllocator() SpanAllocator {
+	return &syncPollSpanAllocator{
+		spanPool: sync.Pool{New: func() interface{} {
+			return &Span{}
+		}},
+	}
+}
+
+func (pool *syncPollSpanAllocator) Get() *Span {
+	return pool.spanPool.Get().(*Span)
+}
+
+func (pool *syncPollSpanAllocator) Put(span *Span) {
+	span.reset()
+	pool.spanPool.Put(span)
+}
+
+type simpleSpanAllocator struct{}
+
+func (pool simpleSpanAllocator) Get() *Span {
+	return &Span{}
+}
+
+func (pool simpleSpanAllocator) Put(span *Span) {
+	// @comment https://github.com/jaegertracing/jaeger-client-go/pull/381#issuecomment-475904351
+	// since finished spans are not reused, no need to reset them
+	// span.reset()
+}