added config_metrics.go which builds on debug tag

When the debug tag is used, the config_metrics.go file
ensures the metrics decorator KV store is imported.

config_metrics.go

@@ -0,0 +1,17 @@
+//  Copyright (c) 2015 Couchbase, 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.
+
+// +build debug
+
+package bleve
+
+import (
+	_ "github.com/blevesearch/bleve/index/store/metrics"
+)
+