sFlow Counter samples are not implemented

[Edward Ting]

Two of the supported sFlow samples are not implemented, SFLOW_COUNTER_SAMPLE and SFLOW_COUNTER_SAMPLE_EXPANDED.

There are 4 sFlow samples defined in sFlow collector, controller/src/analytics/sflow.h, as below.
enum SFlowSampleType {
    SFLOW_FLOW_SAMPLE = 1,
    SFLOW_COUNTER_SAMPLE = 2,
    SFLOW_FLOW_SAMPLE_EXPANDED = 3,
    SFLOW_COUNTER_SAMPLE_EXPANDED = 4
};

However 'controller/src/analytics/sflow_parser.cc' only supports SFLOW_FLOW_SAMPLE and SFLOW_FLOW_SAMPLE_EXPANDED, where SFLOW_COUNTER_SAMPLE and SFLOW_COUNTER_SAMPLE_EXPANDED are missing.