Add Flex counters bulk Dash meter stats fetch support in syncd (#1460)

This PR adds syncd FlexCounter support to periodically fetch
SAI bulk stats for Dash meter buckets and write the stats to
COUNTERS_DB.
Multiple Meter buckets, each with inbound and outbound byte counters,
are internally allocated for each ENI object in the Dash pipeline.
To support this a new DashMeterCounterContext class is derived from
BaseCounterContext that tracks and invokes SAI bulkGetStats for all the
meter bucket objects for each ENI object added to the context.

Reference -
https://github.com/sonic-net/DASH/blob/main/documentation/metering/metering.md

meta/SaiSerialize.cpp

@@ -1096,6 +1096,14 @@ std::string sai_serialize_eni_stat(
     return sai_serialize_enum(counter, &sai_metadata_enum_sai_eni_stat_t);
 }
 
+std::string sai_serialize_meter_bucket_entry_stat(
+        _In_ const sai_meter_bucket_entry_stat_t counter)
+{
+    SWSS_LOG_ENTER();
+
+    return sai_serialize_enum(counter, &sai_metadata_enum_sai_meter_bucket_entry_stat_t);
+}
+
 std::string sai_serialize_tunnel_stat(
         _In_ const sai_tunnel_stat_t counter)
 {

meta/sai_serialize.h

@@ -134,6 +134,9 @@ std::string sai_serialize_buffer_pool_stat(
 std::string sai_serialize_eni_stat(
         _In_ const sai_eni_stat_t counter);
 
+std::string sai_serialize_meter_bucket_entry_stat(
+        _In_ const sai_meter_bucket_entry_stat_t counter);
+
 std::string sai_serialize_tunnel_stat(
         _In_ const sai_tunnel_stat_t counter);