storage: Implement Close() in REST client (#6826)

Calling /minio/prometheuses/metrics calls xlSets.StorageInfo() which creates a new
storage REST client and closes it. However, currently, closing does nothing
to the underlying opened http client.

This commit introduces a closing behavior by calling CloseIdleConnections
provided by http.Transport upon the initialization of this latter.

cmd/storage-rest-client.go

@@ -322,6 +322,7 @@ func (client *storageRESTClient) RenameFile(srcVolume, srcPath, dstVolume, dstPa
 // Close - marks the client as closed.
 func (client *storageRESTClient) Close() error {
 	client.connected = false
+	client.restClient.Close()
 	return nil
 }