Allow filtering dnstap signal in DNS module by the Message_Type protobuf:

enum Message_Type : int {
  Message_Type_AUTH_QUERY = 1,
  Message_Type_AUTH_RESPONSE = 2,
  Message_Type_RESOLVER_QUERY = 3,
  Message_Type_RESOLVER_RESPONSE = 4,
  Message_Type_CLIENT_QUERY = 5,
  Message_Type_CLIENT_RESPONSE = 6,
  Message_Type_FORWARDER_QUERY = 7,
  Message_Type_FORWARDER_RESPONSE = 8,
  Message_Type_STUB_QUERY = 9,
  Message_Type_STUB_RESPONSE = 10,
  Message_Type_TOOL_QUERY = 11,
  Message_Type_TOOL_RESPONSE = 12,
  Message_Type_UPDATE_QUERY = 13,
  Message_Type_UPDATE_RESPONSE = 14
};

As a dnstap user, I want to be able to create policies which extract DNS metrics per "type" defined above: Authoritative, Resolver, Client, Forwarder, Stub, Tool or Update.

Therefor DNS handler needs a new filter dnstap_msg_type which can be set to one of "auth", "resolver", "client", "forwarder", "stub", "tool" or "update" to filter out all dnstap messages except those which match the given type (which will match both QUERY and RESPONSE of that type).