stats: counters data model

stats: counters data model

This adds a new data model for counters.
Specifying the errors severity and unit.
A later patch will update vpp_get_stats to take advantage of this.
Only the map plugin is updates as an example.

New .api language:
A new "counters" keyword to define counter sets.

counters map {
  none {
    severity info;
    type counter64;
    units "packets";
    description "valid MAP packets";
  };
  bad_protocol {
    severity error;
    type counter64;
    units "packets";
    description "bad protocol";
  };
};

Each counter has 4 keywords. severity, which is one of error, info or warn.
A type, which is one of counter64 or gauge64.
units, which is a text field using units from YANG.

paths {
  "/err/ip4-map" "map";
  "/err/ip6-map" "map";
  "/err/ip4-t-map" "map";
  "/err/ip6-t-map" "map";
};

A new paths keyword that maps the counter-set to a path in the stats segment KV store.

Updated VPP CLI to include severity so user can see error counter severity.
DBGvpp# show errors
   Count               Node                    Reason        Severity
        13        ethernet-input              no error         error

Type: feature
Signed-off-by: Ole Troan <ot@cisco.com>
Change-Id: Ib2177543f49d4c3aef4d7fa72476cff2068f7771
Signed-off-by: Ole Troan <ot@cisco.com>