Support yang model for bulk size per counter (#21783)

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#### Why I did it

Support yang model for bulk size per counter

HLD: sonic-net/SONiC#1864

##### Work item tracking
- Microsoft ADO **(number only)**:

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Author: mssonicbld

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

@@ -163,6 +163,7 @@
                 "PORT": {
                     "FLEX_COUNTER_STATUS": "enable",
                     "BULK_CHUNK_SIZE": 100,
+                    "BULK_CHUNK_SIZE_PER_PREFIX": "SAI_PORT_STAT_IF_OUT_QLEN:0;SAI_PORT_STAT_IF_IN_FEC:32",
                     "POLL_INTERVAL": 1000
                 },
                 "PORT_BUFFER_DROP": {

src/sonic-yang-models/yang-models/sonic-flex_counter.yang

@@ -47,6 +47,11 @@ module sonic-flex_counter {
                 }
             }
 
+            typedef bulk_chunk_size_per_prefix {
+                type string;
+                description "Bulk chunk size per counter name prefix";
+            }
+
             description "FLEX_COUNTER_TABLE part of config_db.json";
 
             /* below are in alphabetical order */
@@ -111,6 +116,9 @@ module sonic-flex_counter {
                 leaf BULK_CHUNK_SIZE {
                     type bulk_chunk_size;
                 }
+                leaf BULK_CHUNK_SIZE_PER_PREFIX {
+                    type bulk_chunk_size_per_prefix;
+                }
             }
 
             container PG_WATERMARK {
@@ -127,6 +135,9 @@ module sonic-flex_counter {
                 leaf BULK_CHUNK_SIZE {
                     type bulk_chunk_size;
                 }
+                leaf BULK_CHUNK_SIZE_PER_PREFIX {
+                    type bulk_chunk_size_per_prefix;
+                }
             }
 
             container PORT {
@@ -143,6 +154,9 @@ module sonic-flex_counter {
                 leaf BULK_CHUNK_SIZE {
                     type bulk_chunk_size;
                 }
+                leaf BULK_CHUNK_SIZE_PER_PREFIX {
+                    type bulk_chunk_size_per_prefix;
+                }
             }
 
             container PORT_RATES {
@@ -169,6 +183,9 @@ module sonic-flex_counter {
                 leaf BULK_CHUNK_SIZE {
                     type bulk_chunk_size;
                 }
+                leaf BULK_CHUNK_SIZE_PER_PREFIX {
+                    type bulk_chunk_size_per_prefix;
+                }
             }
 
             container QUEUE {
@@ -185,6 +202,9 @@ module sonic-flex_counter {
                 leaf BULK_CHUNK_SIZE {
                     type bulk_chunk_size;
                 }
+                leaf BULK_CHUNK_SIZE_PER_PREFIX {
+                    type bulk_chunk_size_per_prefix;
+                }
             }
 
             container QUEUE_WATERMARK {
@@ -201,6 +221,9 @@ module sonic-flex_counter {
                 leaf BULK_CHUNK_SIZE {
                     type bulk_chunk_size;
                 }
+                leaf BULK_CHUNK_SIZE_PER_PREFIX {
+                    type bulk_chunk_size_per_prefix;
+                }
             }
 
             container RIF {