[VOL-2570]Make skew configurable for performance metrics

Change-Id: Ifa39408db686dc1cf38a0c8891632d14d1a70625
diff --git a/protos/voltha_protos/device.proto b/protos/voltha_protos/device.proto
index 1341dac..0baf1bc 100644
--- a/protos/voltha_protos/device.proto
+++ b/protos/voltha_protos/device.proto
@@ -67,6 +67,7 @@
     bool freq_override = 4 [(access) = READ_ONLY];
     repeated PmGroupConfig groups = 5; // The groups if grouped is true
     repeated PmConfig metrics = 6; // The metrics themselves if grouped is false.
+    uint32 max_skew = 7; //Default value is set to 5 seconds
 }
 
 // Describes instance of software image on the device