[DPB][YANG] extended yang-model - added 'buffer_model' field, added test cases (#6464)

- Why I did it
The fix for the issue [DPB][YANG] sonic-device_metadata.yang is not aligned with newest changes in CONFIG_DB

- How I did it
CONFIG_DB was extended with the field buffer_model - added representation of this field inside the sonic-device_metadata.yang

- How to verify it
Run the command config interface breakout <interface> <breakout_mode>
Signed-off-by: Vadym Hlushko <vadymh@nvidia.com>

src/sonic-yang-models/tests/files/sample_config_db.json

@@ -202,7 +202,8 @@
                 "mac": "00:11:22:33:dd:5a",
                 "hostname": "asw.dc",
                 "bgp_asn": "64850",
-                "hwsku": "Stone"
+                "hwsku": "Stone",
+                "buffer_model": "dynamic"
             }
         },
         "VLAN": {

src/sonic-yang-models/tests/yang_model_tests/tests/device_metadata.json

@@ -32,5 +32,15 @@
     "DEVICE_METADATA_TYPE_INCORRECT_PATTERN": {
 	"desc": "DEVICE_METADATA_TYPE_INCORRECT_PATTERN pattern failure.",
 	"eStrKey" : "Pattern"
+    },
+    "DEVICE_METADATA_CORRECT_BUFFER_MODEL_PATTERN": {
+        "desc": "DEVICE_METADATA correct value for BUFFER_MODEL field"
+    },
+    "DEVICE_METADATA_CORRECT_BUFFER_MODEL_PATTERN2": {
+        "desc": "DEVICE_METADATA correct value for BUFFER_MODEL field"
+    },
+    "DEVICE_METADATA_INCORRECT_BUFFER_MODEL_PATTERN": {
+        "desc": "DEVICE_METADATA wrong value for BUFFER_MODEL field.",
+        "eStr": ["pattern", "does not satisfy"]
     }
 }

src/sonic-yang-models/tests/yang_model_tests/tests_config/device_metadata.json

@@ -90,5 +90,32 @@
                 }
             }
         }
+    },
+    "DEVICE_METADATA_CORRECT_BUFFER_MODEL_PATTERN": {
+        "sonic-device_metadata:sonic-device_metadata": {
+             "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                "buffer_model": "dynamic"
+                }
+            }
+        }
+    },
+    "DEVICE_METADATA_CORRECT_BUFFER_MODEL_PATTERN2": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                "buffer_model": "traditional"
+                }
+            }
+        }
+    },
+    "DEVICE_METADATA_INCORRECT_BUFFER_MODEL_PATTERN": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                "buffer_model": "incorrect_pattern"
+                }
+            }
+        }
     }
 }

src/sonic-yang-models/yang-models/sonic-device_metadata.yang

@@ -89,6 +89,18 @@ module sonic-device_metadata {
                         pattern "ToRRouter|LeafRouter|SpineChassisFrontendRouter|ChassisBackendRouter|ASIC";
                     }
                 }
+
+                leaf buffer_model {
+                    description "This leaf is added for dynamic buffer calculation.
+                                The dynamic model represents the model in which the buffer configurations,
+                                like the headroom sizes and buffer pool sizes, are dynamically calculated based
+                                on the ports' speed, cable length, and MTU. This model is used by Mellanox so far.
+                                The traditional model represents the model in which all the buffer configurations
+                                are statically configured in CONFIG_DB tables. This is the default model used by all other vendors";
+                    type string {
+                        pattern "dynamic|traditional";
+                    }
+                }
             }
             /* end of container localhost */
         }