Refactor docs; Add 'ipng_source_tag', add udp listener for nginx-ipng-stats plugin

cmd/frontend/frontend_test.go

@@ -229,8 +229,17 @@ func TestDrillURL(t *testing.T) {
 	if !strings.Contains(u, "f_asn=12345") {
 		t.Errorf("drill from asn: missing f_asn in %q", u)
 	}
+	if !strings.Contains(u, "by=source_tag") {
+		t.Errorf("drill from asn: expected next by=source_tag in %q", u)
+	}
+
+	p.GroupByS = "source_tag"
+	u = p.drillURL("direct")
+	if !strings.Contains(u, "f_source_tag=direct") {
+		t.Errorf("drill from source_tag: missing f_source_tag in %q", u)
+	}
 	if !strings.Contains(u, "by=website") {
-		t.Errorf("drill from asn: expected cycle back to by=website in %q", u)
+		t.Errorf("drill from source_tag: expected cycle back to by=website in %q", u)
 	}
 }