pkg/tsdb/stackdriver/stackdriver.go: Fix regular expression does not contain any meta characters.

See,
$ gometalinter --vendor --deadline 10m --disable-all --enable=megacheck ./...
pkg/tsdb/stackdriver/stackdriver.go:171:26:warning: regular expression does not contain any meta characters (SA6004) (megacheck)

pkg/tsdb/stackdriver/stackdriver.go

@@ -168,8 +168,7 @@ func reverse(s string) string {
 }
 
 func interpolateFilterWildcards(value string) string {
-	re := regexp.MustCompile("[*]")
-	matches := len(re.FindAllStringIndex(value, -1))
+	matches := strings.Count(value, "*")
 	if matches == 2 && strings.HasSuffix(value, "*") && strings.HasPrefix(value, "*") {
 		value = strings.Replace(value, "*", "", -1)
 		value = fmt.Sprintf(`has_substring("%s")`, value)