fix(compute/metadata): set IdleConnTimeout for http.Client

This helps clean up idled connections held by the keep-alive.
The default transport set this to 90 seconds but since we
declare our own the default zero value means they are never
closed out if they are under the threshold for connections to
keep alive(2).

Fixes: googleapis#5430

compute/metadata/metadata.go

@@ -70,6 +70,7 @@ func newDefaultHTTPClient() *http.Client {
 				Timeout:   2 * time.Second,
 				KeepAlive: 30 * time.Second,
 			}).Dial,
+			IdleConnTimeout: 60 * time.Second,
 		},
 		Timeout: 5 * time.Second,
 	}