logging: move trace example to build-tagged file

Avoids breaking on Go < 1.8.

Change-Id: I9245008662b18a07585a718653e381fdd622f179
Reviewed-on: https://code-review.googlesource.com/c/34030
Reviewed-by: Jean de Klerk <deklerk@google.com>
Reviewed-by: kokoro <noreply+kokoro@google.com>
diff --git a/logging/examples_go18_test.go b/logging/examples_go18_test.go
new file mode 100644
index 0000000..a678b8d
--- /dev/null
+++ b/logging/examples_go18_test.go
@@ -0,0 +1,39 @@
+// Copyright 2016 Google LLC
+//
+// 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.
+
+// +build go1.8
+
+package logging_test
+
+import (
+	"cloud.google.com/go/logging"
+	"go.opencensus.io/trace"
+	"golang.org/x/net/context"
+)
+
+// This example shows how to create a Logger that disables OpenCensus tracing of the
+// WriteLogEntries RPC.
+func ExampleContextFunc() {
+	ctx := context.Background()
+	client, err := logging.NewClient(ctx, "my-project")
+	if err != nil {
+		// TODO: Handle error.
+	}
+	lg := client.Logger("logID", logging.ContextFunc(func() (context.Context, func()) {
+		ctx, span := trace.StartSpan(context.Background(), "this span will not be exported",
+			trace.WithSampler(trace.NeverSample()))
+		return ctx, span.End
+	}))
+	_ = lg // TODO: Use lg
+}
diff --git a/logging/examples_test.go b/logging/examples_test.go
index d4a2c3a..b4bb452 100644
--- a/logging/examples_test.go
+++ b/logging/examples_test.go
@@ -20,7 +20,6 @@
 	"os"
 
 	"cloud.google.com/go/logging"
-	"go.opencensus.io/trace"
 	"golang.org/x/net/context"
 )
 
@@ -165,19 +164,3 @@
 	fmt.Println(sev)
 	// Output: Alert
 }
-
-// This example shows how to create a Logger that disables OpenCensus tracing of the
-// WriteLogEntries RPC.
-func ExampleContextFunc() {
-	ctx := context.Background()
-	client, err := logging.NewClient(ctx, "my-project")
-	if err != nil {
-		// TODO: Handle error.
-	}
-	lg := client.Logger("logID", logging.ContextFunc(func() (context.Context, func()) {
-		ctx, span := trace.StartSpan(context.Background(), "this span will not be exported",
-			trace.WithSampler(trace.NeverSample()))
-		return ctx, span.End
-	}))
-	_ = lg // TODO: Use lg
-}